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The Greek Concept of Atomos: The Indivisible Atom
I continue to grow in my knowledge. Atomistic theory is prominent in some of the Hindu
teachings in India.
Around 440 BC, Leucippus of Miletus, in his lost book "The Greater World System,"
originated the atom concept. He and his pupil, Democritus (c460-371 BC) of Abdera,
refined and extended it in future years. There are five major points to their atomic idea.
Almost all of the original writings of Leucippus and Democritus are lost. About the only
sources we have for their atomistic ideas are found in quotations of other writers.
Democritus is known as the "Laughing Philosopher"
because of his joyous spirit. He was a big man
(relatively speaking) and enjoyed life tremendously.
He also was very widely traveled, having reportedly
visited Athens.
One point: teachers often think that Democritus
developed the atom concept. This is incorrect. In fact,
Democritus wrote his version in a (now lost) book
called "Little World System." More than likely, he
titled it so out of deference to his teacher.
So, be prepared for your teacher to want Democritus
to be the correct answer. Want some advice? Don't
argue with your teacher based on what some guy on
the Internet said.
This map shows the important towns
of Greece, Turkey and Asia Minor
around the time the atom concept was
developed. It is about 250 miles as the
crow flies between the Abdera and
Miletus.
At this time Greek philosophy was about 150 years old, having emerged early in the sixth
century BC, centered in the city of Miletus on the Ionian coast in Asia Minor (now
Turkey). The earliest known Greek philosopher was Thales of Miletus.
The work of Leucippus and Democritus was further developed by Epicurus (341-270 BC)
of Samos, who made the ideas more generally known. Aristotle (384-322 BC) quotes
both of them extensively in arguing against their ideas. Much of what we know about
their ideas comes to us in a poem titled "De Rerum Natura" (On the Nature of Things)
written by Lucretius (c95-55 BC). This poem, lost for over 1000 years, was rediscovered
in 1417.
On the left is Aristotle and to the right is
Epicurus.
Point #1 - All matter is composed of atoms, which are bits of matter too small to be
seen. These atoms CANNOT be further split into smaller portions.
Democritus quotes Leucippus: "The atomists hold that splitting stops when it reaches
indivisible particles and does not go on infinitely."
In other words, there is a lower limit to the division of matter beyond which we cannot
go. Atoms were impenetrably hard, meaning they could not be divided. In Greek, the
prefix "a" means "not" and the word "tomos" means cut. Our word atom therefore comes
from atomos, a Greek word meaning uncuttable.
Democritus reasoned that if matter could be infinitely divided, it was also subject to
complete disintegration from which it can never be put back together. However, matter
can be reintegrated.
Even though matter can be destroyed by repeated splitting, new things can be made by
joining simpler pieces of matter together. The process of disintegration & reintegration is
reversible.
The idea of reversibility means that there must be a lower limit to the splitting of matter.
If matter can be split infinitely, there is nothing to stop it from going on forever and
destroying all matter.
Only with a definite and finite lower limit to splitting do we keep a permament
foundation of ultimate particles with which to build up everything we see. As Epicurus
says:
"Therefore, we must not only do away with division into smaller and smaller parts to
infinity, in order that we may not make all things weak, and so in the composition of
aggregate bodies be compelled to crush and squander the things that exist into the nonexistent...."
Epicurus also insisted on an upper limit for atoms - they are always invisible. Although
no reason is given, it seems obvious enough: all matter that can be seen by humans is still
divisible, therefore cannot be atoms.
Point #2 - There is a void, which is empty space between atoms.
Aristotle quotes Leucippus: "Unless there is a void with a separate being of its own, 'what
is' cannot be moved-nor again can it be 'many', since there is nothing to keep things
apart."
In other words, there is empty space between atoms. In modern times, we would use the
word vacuum, although the Greeks did not.
Given that all matter is composed of atoms (the ultimate and unchanging particles), then
all changes must be as a result of the movement of atoms. However, in order to move
there must be a void--a space entirely empty of matter--through which atoms can move
from place to place.
Aristotle was opposed to the idea of the void and he based it on his concept of motion,
today called the Aristolelian law of motion. This law held that the velocity of a body was
directly proportional to the motive power and inversely proportional to the resistance of
the medium the body was moving through. Another way to express this: the velocity of a
body is proportional to the force acting on it divided by the resisting force of the medium.
What this means is that, as the medium the body is passing through becomes more and
more "void-like," there is progressively less and less resisting force. Therefore, the body
moves faster and faster, because the resistance (remember, it is in the denominator)
becomes smaller and smaller. In this example, assume that the motive force remains
constant.
Since the void, as conceived by Leucippus and Democritus, was completely empty, there
was zero resistance and the moving speed of the body became infinite. Since, as Aristotle
maintained, an infinite speed was impossible, there could be no void. By the way,
Aristotle's ideas of motion were incorrect. It would not be until Issac Newton in 1687 that
the correct laws of motion were given.
Point #3 - Atoms are completely solid.
It then follows that there can be no void inside an atom itself. Otherwise an atom would
be subject to changes from outside and could disintegrate. Then, it would not be an atom.
We know this is incorrect. In 1911, Ernest Rutherford discovered the nucleus,
demonstrating in the process that a single atom is mostly empty space.
Point #4 - Atoms are homogeneous, with no internal structure.
The absolute solidity of the atoms also leads to the notion that atoms are homogeneous,
or the same all the way through. Another way to express this is that an atom would have
no internal structure.
Although there was speculation about sub-atomic structure in the 1800's after John
Dalton introduced the atom idea on a solid scientific basis, it was not until 1897 and J.J.
Thomson's discovery of the electron that the atom was shown to have an internal
structure.
Point #5 - Atoms are different in ...
1) ...their sizes. See the Democritus quote just below.
2) ...their shapes. According to Aristotle: "Democritus and Leucippus say that there are
indivisible bodies, infinite both in number and in the varieties of their shapes...."
Democritus says of atoms: "They have all sorts of shapes and appearences and different
sizes.... Some are rough, some hook-shaped, some concave, some convex and some have
other innumerable variations."
3) ...their weight. Again from Aristotle: "Democritus recognized only two basic
properties of the atom: size and shape. But Epicurus added weight as a third. For,
according to him, the bodies move by necessity through the force of weight."
Concluding Remarks
The idea of the atom was strongly opposed by Aristotle and others. Because of this, the
atom receeded into the background. Although there is a fairly continuous pattern of
atomistic thought through the ages, only a relative few scholars gave it much thought.
Due to complex circumstances beyond the scope of this lesson, the Catholic Church
accepted Aristotle's position and came to equate atomistic ideas with Godlessness. For
example, "Democritus of Abdera said that there is no end to the universe, since it was not
created by any outside power."
It was not until 1660 that Pierre Gassendi succeeded in separating the two and not until
1803 that John Dalton put the atom on a solid scientific basis. The atom concept is often
presented as laying fallow between Democritus and Dalton.
Atomic Structure from Democritus to Dalton
In 1803, John Dalton of England introduced the atomic idea to chemistry (and is called
the Father of Modern Atomic Theory for his efforts). However, it would be false to
assume that atomic ideas disappeared completely from the intellectual map for over 2000
years. For, although atomic thinkers between the Greeks and Dalton were few, there is a
fairly continuous line from the Greeks to John Dalton.
Much of the following is based on these articles:
1) "The Origins of the Atomic Theory" by J.R. Partington. Annals of
Science, vol 4, no. 3 (July, 1939)
2) "The Atomic View of Matter in the XVth, XVIth, and XVIIth
Centuries" by G.B. Stones. Isis, vol. 10, part 2, No. 34 (January 1928).
I. Atomism in Antiquity
The atomic ideas of Leucippus and Democritus (from about 440 BC) were opposed by
Aristotle about 100 years or so later. Those who acknowledged Aristotle as their master
opposed atoms. Since Epicurus was an atomist, he was opposed by his rivals, the Stoics.
Cicero, Seneca and Galen all spoke against atoms.
Hero of Alexandria (150 A.D.?) makes use of atoms to explain compression and
rarefaction (to thin something out; become less dense). Hero denied the existence of an
extended vacuum, but allowed for a vacuum between atoms. One proof he cited was that
fire could enter into a material, showing that it had openings, i.e., a vacuum. He pointed
out that the pores of a diamond were too small to let in fire and so the stone was
incombustible. (In the 1700's, both Lavosier and Priestly were able to burn diamonds
with large lenses that concentrated the sunlight.)
Important figures within the Church spoke against atoms. Dionysios (Bishop of
Alexandria 200 A.D.), Lactantius (died 324 A.D.) and Augustine (354-430 A.D.) are
names cited by Partington.
II. Atomism in the Middle Ages
Isidore, Bishop of Seville (560-636), the Venerable Bede (672-735), and Hrabanus
Maurus (776-856) all used the word "atom" to refer to discontunities in bodies. William
of Conches (1080-1154) and Vincent of Beauvais (died ca. 1264-8) both showed
knowledge of atomic thinking in their writing. William openly taught about the ideas of
Democritus. Vincent wrote a great encyclopedia, but only gave short quotations about
atoms.
Giles of Rome (ca. 1247-1316) taght that there are natural minima below which physical
substances cannot exist. This implies an atomic theory of matter. He also investigated the
nature of the vacuum using a clepsydra (a water clock) and a siphon, showing that the
void exerted a force of suction.
The works of Aristotle were rediscovered by Western Europe about 1200, in Latin
translations of Arabic translations from the original Greek. Much scholastic discussion
followed among such people as St. Thomas Aquinas (1225-74) and Roger Bacon (121492). Over time, the Catholic Church began to elevate Aristotle's writings to the same
level as Scripture and had associated atomic thinking with Godlessness. (Quite frankly,
the Chemist does not know how the process took place, but it did. On a side issue, the
Church also did the same thing with Ptolemy in astronomy. When Galileo opposed the
Church (in the 1630's?), he was found guilty of heresy. Only recently (around the late
1980's-early 1990's) has the Church formally admitted its error.)
De Rerum Natura was rediscovered in 1417 (and printed in 1473, reprinted in 1486) and
became the prime source (still true today) for the ideas of Leucippus and Democritus.
You may ask how William of Conches knew of Democritus. Scattered about the libraries
of churches in Europe were a few copies of De Rerum Natura. Stones, in his article, cites
three known to have existed in William's lifetime. Other copies certainly also existed at
that time.
III. Atomism in the Renaissance
A) Nicholas of Cusa (1401-1464) wrote:
"What dost thou understand by an atom?
"Under mental consideration that which is continuous becomes divided
into the ever divisible, and the multitude of parts progresses to infinity.
But by actual division we arrive at an actually indivisible part which I call
an atom. For an atom is a quantity, which on account of its smallness is
actually indivisible."
B) Girolamo Fracastoro (1478-1553) was a physician who wrote about atomism. In fact,
the phrase "seeds of disease" is asociated with his name. In discussion the mechanism of
infection, he supposed the existence of minute indivisible substances which convey the
disease. he called these semina. Interestingly, Lucretius (in Book VI) refers to seeds
helpful to life and seeds which cause disease and death. In a different book, Fracastoro
indicates his agreement with Democritus and puts forward an atomistic point of view
concerning chemical reactions.
C) Peter Ramus (1515-1572) broke with Aristotle early in his life. (Remember, the
Catholic Church had long ago elevated Aristotle's works to Scripture. In essence, both
were considered to be infallible.) At age 21, he presented a thesis based on this idea: "all
that Aristotle has said is false." His opponents could not just appeal to the authority of
Aristotle to refute him, since that would be begging the question. After attacking his ideas
for a whole day and being refuted, Ramus was finally awarded his degree with honors.
In 1543, he wrote two books (aganist Aristotle) that provoked violent reaction. Their
publication was banned, the books were burned, and Ramus was silenced by order of the
Pope, Francis I. After the Pope died a year later, Ramus resumed teaching and was
appointed professor in 1551.
However, he embraced the Reformed faith (Martin Luther had nailed his "95 Theses" to
church door at the University of Wittenberg on October 31, 1517.) and was forced to flee
from Paris. His home was pillaged and his library burned. He returned eventually, but
ultimately died in the massacre of St. Bartholomew in Paris in 1572.
Although it appears that Ramus did not write about atomism as such, he was in the
forefront of the attack on the authority of Aristotle.
D) In 1588, Giordano Bruno wrote:
"The division of natural things has a limit; an indivisible something exists.
The division of natural things attains the smallest and last parts which are
not perceptible by the aid of human instruments."
E) Partington lists five other names of people alive through in the 1500's and 1600's who
wrote about atoms. Of interest is Sebastin Basso, who wrote of particles of the first,
second, and third order, that is to say, structures BUILT UP by bringing atoms together.
What we might call a molecule today. J.C. Magnenus attempted to calculate the size of an
atom.
F) Daniel Sennert (1572-1637) was an atomist during the time Rene Descartes (15961650) and Francis Bacon (1561-1626) were alive. Both Bacon and Descartes, although
intellectual giants of that era, were not too convinced about atomism.
Sennert taught that there must be atoms of more than one type and that atoms joined
together to form composite bodies (I think he called these secondary atoms, but I am not
sure). He used the fact that vapor from wine penetrated 4 layers of paper to show the
smallness of atoms. Another example was that a large volume of vapor yielded a small
drop of liquid.
He also taught that atoms retain their essential form. For example, melt some pure gold
and pure silver together until completely mixed. On treating the mixture with nitric acid,
the silver is dissolved and the gold remains.
G) Partington dates the real beginning of the revival of atomic thinking to the invention
of the barometer in 1634 by Evangalestia Torricelli. Above the mercury of the barometer
was a vacuum. An important position of Aristotle (and the Church) was that the vacuum
did not exist. This invention (and the air pump by Otto von Guericke in 1654) dealt a
severe, if not crippling, blow to the non-existence of the vacuum.
IV. Pierre Gassendi (1592-1655)
Gassendi is considered by many to be the reviver of atomism, but as you have seen,
atomism never really went away, it was just on the fringes. However, Gassendi was
successful in making atomism more widely known and acceptable, especially by
separating a belief in atomism from athesism.
Before going into his teachings, it is interesting to note that in 1624, the Parliment of
Paris had issued a decree that anyone holding or teaching a position opposed to Aristotle
(including atomism) was liable to be put to death. Gassendi has influential friends, so he
was left alone.
In 1649 he published his major work on atomism: Syntagma philosophiae Epicuri. It is
divided into three sections: Logic, Physics, and Ethics.
Before even discussing atoms, Gassendi devotes three chapters to discussing the void and
its necessity. He dwells on Torricelli and his experiments at length.
He describes the Greek position: atoms cannot be created nor destroyed, they are solid,
they have weight, and cannot be subdivided. Gassendi taught that atoms are not just
geometric points, but that they have a definite size, although it is very small.
However, he differs from the Greeks in that atoms have not been in existence forever, but
were made by God. The atoms move not a se ipsis (of themselves), but Dei gratia (as a
gift of God). This is the idea which freed atomism from athesism.
Gassendi allows for the union of atoms to form groups, which he calls moleculae or
corpuscula. However, these groups are not held together by attractive forces, but by
mechanical forces such as hooks-and-eyes or antlers.
V. From Gassendi to Dalton: Just Under 150 years
Robert Boyle (1627-91) was an atomist, although he liked the word "corpuscle." In 1661,
published the "Sceptical Chymist." In it, he insists that the chemical elements must be
actual, physical substances rather than the "principles" the alchemists thought of (the
"principle of salt", the "principle of gold" and so on). Boyle says:
"I can easily enough sublime gold into the form of red Chrystalls of a
considerable length; and many other ways may Gold be disguis'd, and help
to constitute Bodies of very different Natures both from It and from one
another, and nevertheless be afterwards reduc'd to the self-same
Numerical, Yellow, Fixt, Ponderous, and Malleable Gold as it was before
its commixture."
Later on in the book, he says of atoms (oops, sorry Bob, corpuscles) of gold:
"though they may not be primary Concretions of the most minute Particles
of matter, but confessedly mixt Bodies, are able to concurre plentifully in
the composition of several very differing bodies without losing their own
Nature or Texture, or having their cohesion violated by the divorce of their
associated parts or ingredients.
Again, he says:
"the difference of Bodies may depend meerly upon that of the schemes
whereinto their Common matter is put . . . so that according as the small
parts of matter reccede from each other, or work upon each other . . . a
Body of this or that denomination is producd."
Incidently, two of the last non-believers in the reality of atoms were Wilhelm Ostwald
and Ernst Mach. (I am not including those who are not in the mainstream of science,
Ostwald and Mach were both respected scientists.) In 1908, Ostwald explicitly stated his
belief in the reality of atoms in the introduction to his textbook Outline of General
Chemistry. In 1915, Mach was still writing in an anti-atomistic way. The following year,
Mach died, aged 78.
Since then, no one of any scientific substance has questioned the reality of atoms.
John Dalton (1766-1844): The Father of the Chemical Atomic Theory
Before delving into Dalton, I want to draw a difference between physical and chemical
atomism.
The path that Dalton took to the chemical atomic theory is complex. As Leonard K. Nash
points out, there are three more-or-less contemporary descriptions of how Dalton
developed his ideas. This includes Dalton's own words on the subject. Add to this the
several theories advanced over the years by historians and there is a lot to read, study and
ponder about. And then, it turns out that the three contemporary descriptions are mutually
contradictory and none are consistent with information available elsewhere.
The Chemist believes that the most satisfactory answer to Dalton's path to the chemical
atomic theory was by way of his studies on vapor pressure, gas solubility and gas
mixtures. I intend to develop this topic in the future.
John Dalton wrote his first table of atomic weights in his notebook dated September
1803. In 1830, in a paper read to the Manchester Literary and Philosophical Society, he
said:
"A series of Essays read before this society and afterwards published in
the 5th Vol. of their Memoirs gradually led me to the consideration of
ultimate particles or atoms & their combinations. Under the date of Sept
3d, 1803, I find in my notebook 'Observations on the ultimate particles of
bodies and their combinations,' in which the atomic symbols I still use
[were] introduced. On the 23rd of October the same year[I r]ead my Essay
on the absorption of Gases [by Water] at the conclusion of which a series
of atomic [weights] was given for 21 simple and compound elements .. . .
"
t was not until 1805 that the above essay was published and it was not until 1808 that
Dalton himself discussed his methods for atomic weight determination. He published his
theories on the atmosphere and gas behavior in a book titled A New System of Chemical
Philosophy. Only in the last few pages (Chapter III) did he discuss his atomic theory.
Modern scholarship has identified four basic ideas in Dalton's chemical atomic theory.
1) chemical elements are made of atoms
2) the atoms of an element are identical in their masses
3) atoms of different elements have different masses
4) atoms only combine in small, whole number ratios such as 1:1, 1:2, 2:3
and so on.
1) elements are made of atoms
Elements are made up of minute, discrete, indivisible, and indestructible particles called
atoms. These atoms maintain their identity through all physical and chemical changes.
This, of course, is not a new idea to Dalton. This basic idea goes back to the Greeks.
However, please keep in mind that atoms, as such, were not part of the chemical
mainstream in the early 1800's.
Dalton's idea of an element is what we believe today - an element is a chemical substance
that cannot be decomposed further by chemical means (i.e. heat, electricity, reacting with
another chemical). This definition traces to Lavoisier.
Daltonian atoms are usually taught as being similar to featureless billiard balls. In truth,
Dalton never ruled out the possibility of subatomic structure. He just knew that the state
of the art in the early 1800's did not allow the physical structure of an atom to be probed.
2) the atoms of an element are identical in their masses
Atoms of the same element have the same properties, such as weight. Atoms of different
elements have different properties, including a different weight. The idea that all atoms of
a given chemical element weigh the same is known today to be incorrect, but in 1803 the
concept of isotopes was just over 100 years in the future.
Also, the concept of chemical combination in 1803 was much, much different than what
Dalton was proposing. Although Dalton was well-known at the time, the most
authoritative chemist of the period was Claude Louis Berthollet and his ideas were
phrased thus:
"Berthollet has shown also, that every body, how weak soever its affinity
for another maybe, is capable of abstracting part of that other from a third,
how strong soever the affinity of that third is, provded it be applied in
sufficient quantity."
Even many years later, Berthollet resisted the idea of the atom: that elements combine in
small, whole number ratios that are fixed. Even into the late 1800's, there were French
chemists who used their authority to punish lesser colleagues and students who publically
supported the chemical atomic theory.
Incidently, please do not think that Berthollet was a total loser. On the contrary. He was
the first, in 1798, to observe a reversible reaction and ideas like the ones expressed in the
above quote worked perfectly well for chemical substances reacting, just not for
determining atomic weights. Some of the negative reaction to his anti-atom stance seems
to have spilled over into unjusly ignoring his other work. Consequently, it was not until
the mid-1860's that chemical equilibrium began to be explored in depth.
3) atoms of different elements have different masses
Although this idea is implicit in Dalton's theory, it is not original with him. Once again,
the Greeks. developed this general idea. This idea was even discussed in chemical
textbooks of Dalton's time, so the idea was "in the air," so to speak.
However, here is where we meet the original contribution of Dalton. Arnold W. Thackray
says (his original is in italics):
"The particular development of Dalton, which distinguishes his chemical
atomic theory from earlier work, was his devising of an effective system
to obtain these relative particle weights from currently available chemical
data. . . ."
In other words, while it was claimed atoms of different elements had different weights,
no one could figure out what the different weight values were. Dalton was the first to do
so.
So, exactly how did he determine atomic weights? I intend to develop this topic in the
future.
4) atoms combine in small, whole number ratios
Chemical combination between two or more atoms occur in simple, numerical ratios (i.e.,
1 to 1, 1 to 2; 2 to 3; etc.).
This point gives immediate explanation to the Law of Definite Proportions, announced by
Joseph Louis Proust in 1797.
During his research, Dalton discovered the Law of Multiple Proportions, another law
which is easily explained by his atomic theory.
Dalton discovered this law while studying some of the oxides of nitrogen.I will defer
discussion of this work to a future time. The law, in modern terminology, is:
Atoms of the same element can unite in more than one ratio with another
element to form more than one compound.
A fifth idea implicit In Dalton's theory, but usually not discussed is this: atoms can be
neither created nor destroyed. An element's atoms do not change into other element's
atoms by chemical reactions. For example, nitrogen and oxygen atoms stay as themselves
even when combined. They can be recovered by decomposing the substance. As Dalton
says:
"We might as well attempt to introduce a new planet into the solar system,
or to annihilate one already in existence, as to create or destroy a particle
of hydrogen. All the changes we can produce, consist in separating
particles that are in a state of cohesion or combination, and joining those
that were previously at a distance."
As with most of Dalton's theory, this idea is not original to Dalton. It is Lavoisier who is
responsible for the Law of Conservation of Mass in chemical reactions.
Finally, a few Dalton factoids to close:
1) In "A New System of Chemical Philosophy," he proposed standard
symbols for the elements. He was the first to do so.
2) 2) He was the first to identify color-blindness. He, himself, suffered
from red-green color-blindness. Still today, "daltonism" is often used
to name this problem.
3) The unit for atomic weight was called a "dalton" for many years. In
modern times, you most often hear it used in biochemical circles, as in
"The atomic weight of that protein is 35,000 daltons."
J.J. Thomson and the Discovery of the Electron
On April 30, 1897, Joseph John (J.J.) Thomson (1856-1940) announced that cathode rays
were negatively charged particles which he called 'corpuscles.' He also announced that
they had a mass about 1000 times smaller than a hydrogen atom, and he claimed that
these corpuscles were the things from which atoms were built up. Later in 1897, he
wrote:
"...we have in the cathode rays matter in a new state, a state in which the subdivision of
matter is carried very much farther than in the ordinary gaseous state: a state in which all
matter-that is, matter derived from different sources such as hydrogen, oxygen, etc.-is of
one and the same kind; this matter being the substance from which the chemical elements
are built up." (J.J. Thomson (1897). "Cathode Rays," Philosophical Magazine 44, 295.)
He had leaped to the conclusion that the particles in the cathode ray (which we now call
electrons) were a fundamental part of all matter. This was reaching quite far beyond what
he had actually discovered. As he was to recall much later:
"At first there were very few who believed in the existence of these bodies smaller than
atoms. I was even told long afterwards by a distinguished physicist who had been present
at my [1897] lecture at the Royal Institution that he thought I had been `pulling their
legs.' " (J.J. Thomson (1936). Recollections and Reflections. G. Bell and Sons: London.
p. 341.)
Thomson's corpuscle hypothesis was not generally accepted, even by British scientists,
until he spoke of it again in 1899. By this time, George Francis FitzGerald (1851-1901),
an Irish physicist, had suggested that Thomson's 'corpuscles' making up the cathode ray
were actually free electrons. In fact, this suggestion was published as a commentary to
the publication of Thomson's April 30, 1897 lecture in which he first announced his
results. Thomson himself continued to use the term corpuscle until 1913.
Other people had measured the e/m ratio or suggested that the cathode rays were
composed of particles, but Thomson was the first to say that the cathode ray was a
building block of the atom. It was a risky thing, but he was proved right and for his
courage he is remembered as the discoverer of the electron.
Walter Kaufmann deserves special mention before leaving this subject. In 1897, he had
better data than Thomson and had it months before him. However, Kaufmann was a
follower of a philosophy called "positivism," championed at that time by Ernst Mach
(whose name is used today to signify the speed of sound - Mach one; twice the
speed of sound - Mach two, and so on). Positivism allowed explanations of
events which were based on sensory experience only. Since submicroscopic
particles were not seen by the human senses, but rather inferred from the data,
Kaufmann could not bring himself to the "corpuscle hypothesis" that Thomson
announced. So it was that Kaufmann missed out on a great discovery and
become a footnote to history. By the way, Kaufmann was convinced by 1901
of the electron's existence and became a leading experimenter working to
determine more about it.
Cathode Ray Tube History
1855 German inventor Heinrich Geissler develops mercury pump - produces first good
vacuum tubes, these tubes, as modified by Sir William Crookes, become the first to
produce cathode rays, leading eventually to the discovery of the electron (and a bit farther
down the road to television).
1858 Julius Plücker shows that cathode rays bend under the influence of a magnet
suggesting that they are connected in some way; this leads in 1897 to discovery that
cathode rays are composed of electrons.
1865 H. Sprengel improves the Geissler vacuum pump. Plücker uses Geissler tubes to
show that at lower pressure, the Faraday dark space grows larger. He also finds that there
is an extended glow on the walls of the tube and that this glow is affected by an external
magnetic field.
1869 J.W. Hittorf finds that a solid body put in front of the cathode cuts off the glow
from the walls of the tube. Establishes that "rays" from the cathode travel in straight
lines.
1871 C.F. Varley is first to publish suggestion that cathode rays are composed of
particles. Crookes proposes that they are molecules that have picked up a negative charge
from the cathode and are repelled by it.
1874 George Johnstone Stoney estimates the charge of the then unknown electron to be
about 10-20 coulomb, close to the modern value of 1.6021892 x 10-19 coulomb. (He used
the Faraday constant (total electric charge per mole of univalent atoms) divided by
Avogadro's Number. James Clerk Maxwell had recognized this method soon after
Faraday had published, but he did not accept the idea that electricity is composed of
particles.) Stoney also proposes the name "electrine" for the unit of charge on a hydrogen
ion. In 1891, he changes the name to "electron."
1876 Eugen Goldstein shows that the radiation in a vacuum tube produced when an
electric current is forced through the tube starts at the cathode; Goldstein introduces the
term cathode ray to describe the light emitted.
1881 Herman Ludwig von Helmholtz shows that the electrical charges in atoms are
divided into definite integral portions, suggesting the idea that there is a smallest unit of
electricity.
1883 Heinrich Hertz shows that cathode rays are not deflected by electrically charged
metal plates, which would seem to indicate (incorrectly) that cathode rays cannot be
charged particles.
1886 Eugen Goldstein observes that a cathode-ray tube produces, in addition to the
cathode ray, radiation that travels in the opposite direction - away from the anode; these
rays are called canal rays because of holes (canals) bored in the cathode; later these will
be found to be ions that have had electrons stripped in producing the cathode ray.
1890 Arthur Schuster calculates the ratio of charge to mass of the particles making up
cathode rays (today known as electrons) by measuring the magnetic deflection of cathode
rays. Joseph John (J.J.) Thomson first becomes interested in the discharge of electricity
through a gas a low pressure, that is to say, cathode rays.
1892 Heinrich Hertz who has concluded (incorrectly) that cathode rays must be some
form of wave, shows that the rays can penetrate thin foils of metal, which he takes to
support the wave hypothesis. Philipp von Lenard develops a cathode-ray tube with a thin
aluminum window that permits the rays to escape, allowing the rays to be studied in the
open air.
1894 J.J. Thomson announces that he has found that the velocity of cathode rays is much
lower than that of light. He obtained the value of 1.9 x 107 cm/sec, as compared to the
value 3.0 x 1010 cm/sec for light. This was in response to the prediction by Lenard that
cathode rays would move with the velocity of light. However, by 1897, he distrusts this
measurement.
Special Note: At this time there was great rivalry between German and British
researchers. As concerning the nature of the cathode ray, the Germans tended to the
explanation that cathode rays were a wave (like light), whereas the British tended to
believe that the cathode ray was a particle. As events unfold over the next few decades,
both will be proven correct.
In fact, J.J. Thomson will be awarded the Nobel Prize in Physics in 1906 for proving the
electron is a particle and his son, George Paget Thomson, will be awarded the Nobel
Prize in Physics in 1937 for showing that the electron is a wave.
1895 Jean-Baptiste Perrin shows that cathode rays deposit a negative electric charge
where they impact, refuting Hertz's concept of cathode rays as waves and showing they
are particles.
1896 Pieter P. Zeeman discovers that spectral lines of gases placed in a magnetic field are
split, a phenomenon call the Zeeman effect; Hendrik Antoon Lorentz explains this effect
by assuming that light is produced by the motion of charged particles in the atom.
Lorentz uses Zeeman's observations of the behavior of light in magnetic field to calculate
the charge to mass ratio of the electron in an atom, a year before electrons are discovered
and 15 years before it is known that electron are constituents of atoms.
J.J. Thomson's Cathode Ray Tube
J.J. Thomson used results from cathode ray tube (commonly abbreviated CRT)
experiments to discover the electron.
The image (23K GIF) below is of J.J. Thomson and a cathode ray tube from around 1897,
the year he announced the discovery of the electron. Only the end of the CRT can be seen
to the right-hand side of the picture.
The image (7K GIF) below of a CRT used by Thomson in his experiments. It is about
one meter in length and was made entirely by hand.
Th diagram (6K GIF) below appeared in an article by J.J. Thomson in 1897 announcing
the discovery of the electron. You may wish to compare it to the photo. The long glass
finger (in the photo) projecting downward from the right-hand globe is where the entire
tube was evacuated down to as good as a vacuum as could be produced, then sealed.
The two plates about midway in the CRT were connected to a powerful electric battery
thereby creating a strong electrical field through which the cathode rays passed. Thomson
also could use magnets, which were placed on either side of the straight portion of the
tube just to the right of the electrical plates. This allowed him to use either electrical or
magnetic or a combination of both to cause the cathode ray to bend.
The amount the cathode ray bent from the straight line using either the electric field or
the magnetic field allowed Thomson to calculate the e/m ratio.
Incidently, Thomson was a very unhandy person. He was very fumble fingered and had a
tendancy to break things. About 1894 he acquired an excellent glassblower named E.
Everett who helped to greatly increase Thomson's experimental range.
General Cathode Ray Tube Results
There were a number of results gathered over the years by cathode ray tube researchers.
1) If an object is placed in the path of the cathode ray, a shadow of the object is cast on
the glowing tube wall at the end. This showed that the cathode rays traveled in straight
lines.
2) The cathode ray can push a small paddle wheel up an incline, against the force of
gravity. This showed that the cathode ray carried energy and could do work.
3) The cathode ray is deflected from a straight line path by a magnetic field, suggesting
that the two were related in some way. The discovery of this effect in 1855 predates by
some ten years the unification of electricity and magnetism by James Clerk Maxwell.
4) Although there was some speculation that the cathode rays were negatively charged, it
is not shown to be true by experiment until 1895, just two years before Thomson
announces the electron.
5) J.J. Thomson is the first individual to succeed in deflecting the cathode ray with an
electrical field. He did so in 1897. The cathode rays bend toward the positive pole,
confirming that cathode rays is negatively charged.
What is the e/m ratio?
e/m ratio stands for charge-to-mass ratio of the electron.
The modern value for the charge on the electron (to four significant places) is 1.602 x 1019
coulombs and the electrons mass is 9.109 x 10-31 kilograms.
Therefore, the modern value for the e/m ratio is 1.759 x 1011 C/kg. Usually, grams are
used rather than kilograms giving a numerical value of 1.759 x 108. Often, books round
off the 1.759 portion to 1.76.
However, there is one problem. Many textbooks and articles use the m/e ratio, that is the
mass-to-charge ratio. Reversing the above figures and using grams rather than kilograms
gives a value of 5.686 x 10-9 g/C.
The e/m ratio is important because that is as far as Thomson could get with his cathode
ray tubes. Knowledge of the value of 'e' or of 'm' would be needed to get to the other once
you knew e/m, which Thomson did know.
Elsewhere you will find discussion of how the value for 'e,' the charge on the electron
was determined. (May 1996: This will be made a link when that section is written.)
Thomson's Calculations
For a fuller discussion of the below, please see "The Discovery of Subatomic Particles"
by Steven Weinberg. It was published in 1983 by W.H. Freeman and Company. The
ISBN is 0-7167-1488-4.
Thomson had developed formulas based on the deflection of the cathode ray by the
electric field and by the magnetic field. Just below are GIFs of each formula.
By carrying out the experiments and measuring the proper values, he could calculate
what the charge-to-mass (e/m) ratio was for the cathode ray. However, the two formulas
above could not give either the charge or the mass by itself. A different experiment would
have to be carried out.
Notice that both equations depend on knowing the velocity of the cathode ray. (Several
years previous to 1897, Thomson had measured the cathode rays' velocity, but he grew to
distrust the results.) However, both equations can be used as a ratio if the deflections by
the two fields are made to be equal. Then, the mass, charge and both lengths cancel,
leaving us with:
Since Thomson knew both the electrical and magnetic field strengths as well as the
amount of deflection, he could easily solve for the velocity. That value was then inserted
along with the other values into the deflection formulas shown above. An easy
calculation gave the charge-to-mass ratio.
The Thomson Model of the Atom
In 1897, J.J. Thomson discovered the electron, the first subatomic particle. He also was
the first to attempt to incorporate the electron into a structure for the atom. The internal
structure of the atom had been a source of speculation for thousands of years. The Greeks
taught that the atom was solid, as did Dalton. Although Dalton did allow for the fact that
there might be a sub-atomic structure of which he was unaware.
Thomson faced two major problems: (1) how to account for the mass of the atom when
the electron was only about 1/1000 the mass of the hydrogen atom (the more modern
figure is 1/1836) and (2) how to create a neutral atom when the only particle available
was negatively charged.
His solution was to rule the scientific world for about a decade and Thomson himself
would make a major contribution to undermining his own model.
I. Leadup to Thomson's 1904 Model of the Atom
Thomson had been in the business of proposing atomic models since at least 1881, which
is when he proposed his "vortex" model of the atom. We will not go into details about it.
The first seed of the model we are discussing appear in his famous 1897 announcement
of the discovery of the electron. He wrote:
"The explanation which seems to me to account in the most simple and
straightforward manner for the facts is founded on a view of the
constitution of the chemical elements which has been favourably
entertained by many chemists: this view is that the atoms of the different
chemical elements are different aggregations of atoms of the same kind. In
the form in which this hypothesis was enunciated by Prout, the atoms of
the different elements were hydrogen atoms; in this precise form the
hypothesis is not tenable, but if we substitute for hydrogen some unknown
primordial substance X, there is nothing known which is inconsistent with
this hypothesis, which is one that has been recently supported by Sir
Norman Lockyer for reasons derived from the study of the stellar spectra.
If, in the very intense electric field in the neighbourhood of the cathode,
the molecules of the gas are dissociated and are split up, not into the
ordinary chemical atoms, but into these primordial atoms, which we shall
for brevity call corpuscles; and if these corpuscles are charged with
electricity and projected from the cathode by the electric field, they would
behave exactly like the cathode rays. "
And a few paragraphs later:
"If we regard the chemical atom as an aggregation of a number of
primordial atoms, . . . ."
However, he does not go into the presence of a positive force, although he must have
been aware of its necessisity.
Here is what he then said in 1899:
"I regard the atom as containing a large number of smaller bodies which I
will call corpuscles, these corpuscles are equal to each other.... In the
normal atom, this assemblage of corpuscles forms a system which is
electrically neutral. Though the individual corpuscles behave like negative
ions, yet when they are assembled in a neutral atom the negative effect is
balanced by something which causes the space through which the
corpuscles are spread to act as if it had a charge of positive electricity
equal in amount to the sum of the negative charges of the corpuscles....
The detached corpuscles behave like negative ions, each carrying a
constant negative charge which we shall call for brevity the unit charge;
while the part of the atom left behind behaves like a positive ion with the
unit positive charge and a mass large compared with that of the negative
ion."
This last portion is interesting in that it proposes the correct mechanism for ionization; a
negative electron is removed leaving behind a positive atom.
II. Thomson's Mature Model
His next statement on the structure of the atom comes in a 1904 article. The first half of
the article is filled with detailed calculations about the stability of corpuscles moving
about in a positive environment. In fact, Thomson is only able to make calculations
where all the corpuscles are limited to roatating in a ring. Moving from ring to sphere
proves too difficult a challenge.
Here is a quote from the 1904 article:
We suppose that the atom consists of a number of corpuscles moving
about in a sphere of uniform positive electrification . . . .
That seems pretty straighforward, but the problem will soon become the electrons and
their mass.
By the way, this is often referred to as Thomson's "plum pudding model," where the
pudding represents the sphere of positive electricity and the bits of plum scattered in the
pudding are the electrons. The Chemist likes to call it the "blueberry muffin" model. All
those round little blueberries surrounded by the bread of the muffin. Ummmm, good.
Some butter on top of a muffin hot from the oven and some nice, COLD milk. Oh my.
However, not everyone is convinced this is the right answer. Savante Arrhenius (the 1903
Nobel Prize winner in Chemistry) had this to say about Thomson's model in 1907:
"This conception has hitherto remained only a formal one, and has led to
no new results."
Arrhenius goes on to several criticisms of the Thomson Model.
Before leaving this topic, I want to make a point about how the Thomson Model is
presented today. Sometimes teachers, and even textbooks, will represent the Thomson
Model as a mixture of protons and electrons, like on the right-hand side of this image:
Make sure you have the correct idea firmly in mind. The Thomson Model has negative
partices (electrons) and a sphere of positive charge. There are NO protons in the
Thomson Model of the atoms. Be careful, a teacher might try to trip you up on a test
question. (Those teachers sure are evil, aren't they??)
The Thomson Model will hold sway for a few years, until Ernest Rutherford announces
the nuclear model of the atom in 1911. Interest in the Thomson Model fell off rapidly
after 1911, although in 1914 and 1915 attempts were made to resurrect it. These efforts
came to nothing and the Thomson Model assumed its place in history as the first modern
attempt to construct a theory of atomic structure.
Rutherford's Experiment - Part I: 1906 to Early 1911
I. January 1906
Rutherford announced the discovery of alpha particle scattering by air in Jan. 1906. He
took a wire coated with radioactive material and passed the alpha rays through a narrow
slit. This resulted in a narrow, rectangular beam rather than a narrow, circular pencilshaped beam.
In vacuum, the narrow slit showed perfectly sharp edges on a photographic plate the
alpha rays hit. However, when the beam was passes through air, the edges of the slit
became diffuse and widened. Rutherford wrote ". . . the greater width and lack of
definition of the air lines show evidence of an undoubted scattering of the rays in their
passage through air."
II. January to June 1906
Rutherford did not work in a vacuum (although most of the experiments were!), but
rather discussed his results with colleagues, including William H. Bragg (who, together
with his son William L. Bragg, would win the 1915 Nobel Prize in Physics). Bragg was
not happy with Rutherford's conclusion and suggested an alternative explanation.
Rutherford's response was to perform a more definitive experiment, publishing the results
in June 1906.
III. June 1906: Scattering by Mica
Rutherford used his wire coated with the alpha-emitting radioactive element, but he
modified the slit the alpha rays traveled through. Half the slit was left open and half was
covered by a thin (0.003 cm) mica plate. The experiment was done in the vacuum; the
mica taking the place of the air. The open side of the resulting photographic image was
sharp and the mica side was diffuse.
He also subjected the alpha beam to a magnetic field. This was in response to Bragg's
alternative explanation which involved electrons (the exact details are not necessary).
Since electrons bend the opposite way from alpha particles, any electrons produced by
the mica as the beam went through it would be swept away. The result? The image was
the same as when there was no magnetic field. The vacuum side was sharp and the mica
side was diffuse.
Rutherford had proved conclusively the alpha particles could be scattered.
IV. More Results from the Mica Experiment
Careful measuring of the images allowed Rutherford to deduce that some alpha particles
had been scattered by 2° from a straight-line path. In light of future events, he makes the
interesting statement that other particles may have been deflected "through a considerably
greater angle."
He also calculates that a field strength of 100 million volts per cm is required to bend the
alpha particles through 2°. In another very interesting statement given future events, he
says this result "brings out clearly the fact that the atoms of matter must be the seat of
very intense electrical forces -- a deduction in harmony with the electronic theory of
matter." By this he meant the Thomson model of the atom.
Keep in mind that the Thomson model (announced in early 1904, but speculated on in
print by Thomson as early as 1899) is still the only model of the atom acceptable to the
British. The "Saturnian model," announced by Nagaoka in May 1904, had been totally
discredited by Thomson and his allies. Or so they thought!! Also, remember that
Rutherford had been Thomson's student and the two were very close. (J.J.'s son George
has many memories, from his childhood, of Rutherford and his wife over for dinner.) So
it is quite natural for Rutherford to believe that J.J.'s model is correct.
V. Moving to Manchester
All this time, Rutherford had been in Canada (since 1898) at McGill University in
Montreal. In 1907, a position came open at the University of Manchester and Rutherford
applied it and was selected. Now he was back in England and, more importantly for our
story, he hooked up with Hans Geiger.
VI. Manchester, 1908
One of the central goals of Rutherford's work was to determine the nature of the alpha
particle. Was it a singly-charged hydrogen molecule or a doubly-charged helium atom?
The e / m ratio was consistent with either. Although in 1907 he was confident it was the
latter, he sought additional confirmation. To this end, while in Canada he had tried an
experiment which needed to count the number of alpha particles. It was a failure.
However, with Geiger now involved, a satisfactory counting device was designed and
built.
However, the scattering of the alpha particle was wreaking havoc on their results. The
counter worked fine, but as Rutherford put it in a letter to a friend, "the scattering is the
devil." It was evident to both Rutherford and Geiger that an accurate picture of alpha
particle scattering was required. Geiger started on this work even before the counting
experiment was done, and in June 1908 he announced some preliminary results.
Above is Geiger's 1908 apparatus. R is the source, S is the thin metal foil which scatters
the particles, Z is the zinc sulfide screen that flashed when struck (it was called the
scintillation method), and M is a microscope to view the flashes with. It almost goes
without saying, this work was done in a very dark room.
As this work proceeded, Geiger began to notice what he termed a "notable" scattering; in
other words he was seeing scattering by greater angles than he had expected. In addition
to this, he found that gold foil scattered through a greater angle than aluminum. As
Geiger put it, "quite an appreciable angle." He proposed to continue with as many metals
as possible "in the hope of establishing some connection between the scattering and
stopping powers of these materials." In other words, what was first a problem now bcame
the subject of study.
VII. Marsden Enters the Scene
To assist in these experiments, twenty-year-old Ernest Marsden joined Geiger. The tube
was improved and lengthened, but experimental difficulties persisted. They could gain
only a rough estimate of the most probable angle of deflection for a given metal. They
(by the early spring of 1909) decided the trouble was somehow related to scattering by
the tube walls and a series of washers installed in the tube solved the problem. According
to Marsden (many years later), neither of them thought the alpha particles were being
directly REFLECTED from the walls or the metal foil.
As Geiger himself says, some thirty years later:
"In the electric counting of alpha-particles it was seen that the small
residuum of gas in the four-metre long tube . . . influenced the result. We
attributed this to a slight scattering of the alpha-particles. Later on, I
examined scattering quantitatively in several experiments. The most
important observation was the appearance of isolated instances of
extremely large angles of deflection, which were far outside the normal
variations. At first we could not understand this at all."
VIII. Rutherford makes the Critical Suggestion
Hans Geiger was a scientist in his own right. He held a Ph.D., was a teacher on staff, and
did research with Rutherford's direction and collaboration. It was at this time that he said
(according to Rutherford), "Don't you think that young Marsden whom I am training in
radioactive methods ought to begin a small research?" Rutherford agreed and, as Marsden
remembers it 50 years later, he came into the lab one day, turned to Marsden and said
"See if you can get some effect from alpha particles directly reflected from a metal
surface." Marsden goes on to say about his own thoughts:
"I did not think he expected any such result, but it was one of those
hunches that perhaps some effect might be observed . . . ."
The year before he died, Rutherford himself recalled what then happened:
"Two or three days later Geiger came to me in great excitement and said:
"We have been able to get some of the alpha particles coming backwards."
For Ernest Rutherford, winner of the 1908 Nobel Prize in Chemistry, his greatest
scientific achievement still lay two years into the future. Sometime in late 1910/early
1911, he switched roles with Geiger, who remembers it from 27 years later:
"One day Rutherford, obviously in the best of spirits, came into my room
and told me that he now knew what the atom looked like."
IX. The Experiment of 1909
Marsden first used the following set-up in his search for reflected alpha particles:
The alpha emitter was placed at A on top of a lead plate which prevented direct access of
the particles to the counter located at S. With nothing placed at position R the counter did
not record any hits. However, when one thin gold foil was placed at R, the counter came
to life.
It was from this experimental set-up that Geiger reported two or thre days later that some
alpha particles had been reflected back.
This is the opening paragraph of Geiger and Marsden's paper of May 1909:
When β-particles fall on a plate, a strong radiation emerges from the same
side of the plate as that on which the β-particles fall. This radiation is
regarded by many observers as a secondary radiation, but more recent
experiments seem to show that it consists mainly of primary β-particles,
which have been scattered inside the material to such an extent that they
emerge again at the same side of the plate. For α-particles a similar effect
has not previously been observed, and is perhaps not to be expected on
account of the relatively small scattering which α-particles suffer in
penetrating matter.
This is the data they reported in that same paper:
3.
2.
4.
Number of scintillations
Atomic weight, A
A/Z.
per minute, Z.
Lead
207
62
30
Gold
197
67
34
Platinum
195
63
33
Tin
119
34
28
Silver
108
27
25
Copper
64
14·5
23
Iron
56
10·2
18·5
Aluminium.
27
3·4
12·5
1.
Metal
For platinum, Geiger and Marsden reported:
"Three different determinations showed that of the incident α-particles
about 1 in 8000 was reflected, under the described conditions."
X. 1909 Fades into 1910 and then 1911
Now, Ernest Rutherford was confronted with the challenge of explaining the results. His
answer, to be published in May 1911, was the nuclear atom and it made Rutherford
unique among all other Nobel Prize winners. You see, almost all historians of science call
the discovery of the nucleus Rutherford's greatest scientific work. He is the only one to
do his greatest work after receiving the Nobel Prize.
Here is what he says in a lecture at Clark University in September, 1909; just 6 months
after the discovery of large-angle scattering:
"Geiger and Marsden observed the suprising fact that about one in eight
thousand α particles incident on a heavy metal like gold is so deflected by
its encounters with the molecules that it emerges again on the side of the
incidence. Such a result brings to mind the enormous intensity of the
electric field surrounding or within the atom."
Notice the word "encounters;" it's plural. Rutherford is thinking about multiple scattering.
In other words, the alpha particle encounters one gold atom after another and each
encounter deflects the alpha a bit more, so that the sum of all the deflections is to make
the alpha particle come flying out 90° or more from the direction it went in.
In 1910, two events important to our story happen.
1) On Feb, 17, 1910, Geiger reads a paper in which he determines the
most likely angle of deflection for any one alpha particle to be about 1°.
However, in the same paper, he says, "It does not appear profitable at
present to discuss the assumption that might be made to account for [it]."
It seems safe to say that 10 months after the discovery of large-angle
scattering, Geiger (and Rutherford) do not have a clue as to its
explanation.
2) J.D. Crowther (a student of J.J. Thomson) publishes a theory of betaparticle scattering in March 1910. He follows up with more data in June
and then December. He uses the 'multiple scattering' idea; that is, many
small deflecions which add up to one large angle. Each deflecion is caused
by the particle encountering an atom. Over time, Rutherford (in discussion
with his friend W.H. Bragg) will become convinced that this is the
incorrect explanation for large angle scattering.
It is not possible to put a precise date on when Rutherford hit on single scattering as the
answer. The papers which bear his calculations are undated. However a series of letters
he wrote at this time allow a glimse into when he started to put everything together. The
very first mention of the atom occurs in a letter dated Dec. 14, 1910 (to B.B. Boltwood,
an American chemist) in which Rutherford says he has been doing "a good deal of
calculation on scattering." Scattering and the atom figure in nine letters between then and
Feb. 19, 1911. To keep these dates in perspective, remember that Rutherford's published
paper appeared in Philosophical Magazine for May 1911.
Just to review, 'single scattering' refers to one encounter of the alpha particle with an
atom. One single incident is sufficient to deflect the alpha more than 90°. It does seem a
bit fantastic that only one encounter with an atom was sufficient, but Rutherford was
having serious problems making multiple scattering fit.
Rutherford's Experiment - Part II: The Paper of 1911
I. What Confronted Rutherford?
Ernest Rutherford had been studying alpha particles since 1898. In fact, he discovered
them. To him, alpha particles were part of the family. In 1909 he was confronted with
some rather bizzare alpha-particle behavior that he had to explain. What was the
behavior, exactly?
Hans Geiger and Ernest Marsden aimed a stream of alpha particles at a thin gold foil for
several months in 1909. (They would continue studying scattering until 1913.) Geiger
cites a thickness of 8.6 x 10¯6 cm. for the foil. In fact, the foil was so thin that it had to be
supported on a glass plate. (The plate without any foil was studied and no deflecions were
found. It was transparent to the alpha particles.) There were three major findings:
1) Almost all of the alpha particles went through the gold foil as if it were
not even there. Those alpha particles, of course, continued on a straightline path until they hit the detector screen.
2) Some of the alpha particles were deflected only slightly, usually 2° or
less. Geiger found that an alpha particle was, on average, deflected about
1/200th of a degree by each single encounter with a gold atom. The most
probable angle of deflection for one gold foil turned out to be about 1°.
(Rutherford cites a figure of 0.87° in his 1911 paper.)
3) A very, very few (1 in 8000 for platinum foil) alpha particles were
turned through an angle of 90° or more. (Rutherford cites 1 in 20,000 for
gold in his 1911 paper.)
This is a diagram incorporating the
three findings. R is the source of
alpha particles and F is the foil that
scatters the alpha particles. M is the
microscope used to look at the
detector screen which was attached to
the front of the microscope.
The flashes on the screen were very
faint, so a very dark room was
required. The person doing the
viewing had to sit in the dark for
about an hour before beginning the
experiment, to ensure maximum eye
sensitivity.
All Rutherford had to do was explain how it all fit together.
II. OK, Get to the Answer, Big Guy
And Rutherford was a big guy. He was fun, outgoing and vigorous, the life of the party -a great, big, over-grown child. He was also a hard-working scientist who loved science
for itself and never tired of playing in the laboratory. He seldom had problems with
people; on one occasion was even able to turn a potential enemy into a co-worker. Upon
meeting people like Einstein, Lorentz, and Planck for the first time, he was able to turn
them into immediate best buddies. He found a real soul-mate in Marie Curie. They both
loved doing pure research, just letting the science take them where it would, with no
purpose other than to discover new and exciting things.
His solution to the enigma of explaining both large- and small-angle scattering, as you
probably know, was the nucleus. It was already well established that the atom had a
radius of about 10¯8 cm. The Thomson model of the atom spread the entire mass of the
atom throughout that space. What Rutherford did was put most of the mass of the atom at
the center of the atom, in a space much, much smaller that the atom itself -- this is the
nucleus.
For the purposes of his 1911 paper, he considered the nucleus to act as a point:
"We shall suppose that for distances less that 10¯12 cm the central charge
and also the charge on the alpha particle may be supposed to be
concentrated at a point."
Rutherford never used the word "nucleus" in his paper. His phrase was "charge
concentration." In 1912, in a book he published, he devotes a few pages to the nuclear
model and uses the word nucleus once.
So, how does the nucleus account for the three major findings by Geiger and Marsden?
1) The nucleus is so small that the odds are overwhelmingly in favor of a
given alpha particle motoring right on through the gold foil as if nothing
were there. It turns out that the atom is a very empty place, indeed!
2) Some alphas, by pure random chance, will pass near some gold atom
nuclei during their passage through the foil and will be slightly deflected.
By pure chance, some or all of the small deflections will add up and shove
the alpha particle off a straight-line path. Those alphas will emerge
slightly deviated (say one or two degrees) from a straight-line path. (It
might be helpful to remember that the gold nucleus and the alpha particle
are both positively charged, so they will repel each other as they come
close together.)
3) A very, very few alphas, by pure, random chance, will hit a nucleus
almost head-on. The alpha, traveling at 10% the speed of light, penetrates
the atom and gets very close to the nucleus. However, the repulsion
between the alpha and the atom nucleus is so great that the atom flings the
alpha back out, and it does so in a hyperbolic path. Depending on various
factors, this occasionally results in the alpha being turned around 90° or
more. The very heavy nucleus recoils a bit from the impact, but essentially
goes nowhere.
III. That's It, Folks
Rutherford closed the door on the basic structure of the atom. No serious challenge has
arisen to the nuclear model of the atom. However, early in his paper, Rutherford writes
the following:
"The question of the stability of the atom proposed need not be considered
at this stage, for this will obviously depend upon the minute structure of
the atom, and on the motion of the constituent charged parts."
Rutherford is not prepared to take the next step, which is to determine how the electrons
are arranged in the atom. However . . .
In March 1912, 27-year-old Niels Bohr (awarded a Ph.D. in May 1911, the same month
of Rutherford's classic paper) will arrive in Rutherford's laboratory, having just spent a
bit more than 6 months in J.J. Thomson's laboratory. Ernest Rutherford has discovered
the nucleus and now it's time to take a well-earned rest.
Determination of the Charge on an Electron
I. Pre-History
The charge on electron was first
measured by J.J. Thomson and two coworkers (J.S.E. Townsend and H.A.
Wilson), starting in 1897. Each used a
slightly different method. Townsend's
work will be described as an example.
Townsend's work depended on the fact
that drops of water will grow around
ions in humid air. Under the influence
of gravity, the drop would fall,
accelerating until it hit a constant speed.
Several items were measured in this experiment.
1. the mass of a water droplet (actually the average mass of many)
2. the total electric charge carried on all the droplets (this was done by
absorbing the water into an acid and measuring the charge picked up.)
3. the velocity of the droplet
4. the total mass of all water droplets (found by measuring the acid's
increase in weight)
He determined the e/m ratio of the droplets (2 divided by 4), then multiplied by the mass
of one droplet to get the value for e.
Thomson, Townsend, and Wilson each obtained roughly the same value for the charge on
positive and negative ions. It was about 1 x 10¯19 coulombs. This work continued until
about 1901 or 1902.
II. Robert A. Millikan's Definitive Measurement
Robert A. Millikan started his work on electron charge in 1906 and continued for seven
years. His 1913 article announcing the determination of the electron's charge is a classic
and Millikan received the Nobel Prize for his efforts.
Here is a diagram of his apparatus, reproduced from his 1913 article:
Here is Millikan's description:
8. THE EXPERIMENTAL ARRANGEMENTS.
The experimental arrangements are shown in Fig. 1. The brass vessel D
was built for work at all pressures up to 15 atmospheres but since the
present observations have to do only with pressures from 76 cm. down
these were measured with a very carefully made mercury manometer M
which at atmospheric pressure gave precisely the same reading as a
standard barometer. Complete stagnancy of the air between the condenser
plates M and N was attained first by absorbing all of the heat rays from the
arc A by means of a water cell w, 80 cm. long, and a cupric chloride cell
d, and second by immersing the whole vessel D in a constant temperature
bath G of gas-engine oil (40 liters) which permitted, in general,
fluctuations of not more than .02° C. during an observation. This constant
temperature bath was found essential if such consistency of measurement
as is shown below was to be obtained. A long search for causes of slight
irregularity revealed nothing so important as this and after the bath was
installed all of the irregularities vanished. The atomizer A was blown by
means of a puff of carefully dried and dust-free air introduced through the
cock e. The air about the drop p was ionized when desired by means of
Röntgen rays from X which readily passed through the glass window g.
To the three windows g (two only are shown) in the brass vessel D
correspond, of course, three windows in the ebonite strip c which encircles
the condenser plates M and N. Through the third of these windows, set at
an angle of about 18° from the line Xpa and in the same horizontal plane,
the oil drop is observed.
This is a photo dating from the time of the experiment.
These are some points to be made about the experiment:
1. The two plates were 16 mm across, "correct to about .01 mm."
2. The hole bored in the top plate was very small.
3. The space between the plates was illuminated with a powerful beam of light.
4. He sprayed oil ("the highest grade of clock oil") with an atomizer that made drops one
ten-thousandth of an inch in diameter.
5. One drop of oil would make it through the hole.
6. The plates were charged with 5,000 volts.
7. It took a drop with no charge about 30 seconds to fall across the opening between the
plates.
8. He exposed the droplet to radiation while it was falling, which stripped electrons off.
9. The droplet would slow in its fall. The drops were too small to see. What he saw was a
shining point of light.
10. By adjusting the current, he could freeze the drop in place and hold it there for hours.
He could also make the drop move up and down many times.
11. Since the rate of ascent (or descent) was critical, he has a highly accurate scale
inscribed onto the telescope used for droplet observation and he used a highly accurate
clock, "which read to 0.002 second."
Millikan's Improvements over Thomson
1. Oil evaporated much slower than water, so the drops stayed essentially constant in
mass.
2. Millikan could study one drop at a time, rather than a whole cloud.
3. In following the oil drop over many ascents and descents, he could measure the drop as
it lost or gained electrons, sometimes only one at a time. Every time the drop gained or
lost charge, it ALWAYS did so in a whole number multiple of the same charge.
The value as of 1991 (for the charge on the electron) is 1.60217733 (49) x 10¯19
coulombs. This is less than 1% higher than the value obtained by Millikan in 1913. The
49 in parenthesis shows the plus/minus range of the last two digits (the 33). It is unlikely
that there will be much improvement of the accuracy in years to come.
Interesting Fact about Robert Millikan's Experiment
In "The Discovery of Subatomic Particles" by Steven Weinberg there appears a footnote
on p. 97. It reads:
. . . . there appeared a remarkable posthumous memoir that throws some
doubt on Millikan's leading role in these experiments. Harvey Fletcher
(1884-1981), who was a graduate student at the University of Chicago, at
Millikan's suggestion worked on the measurement of electronic charge for
his doctoral thesis, and co-authored some of the early papers on this
subject with Millikan. Fletcher left a manuscript with a friend with
instructions that it be published after his death; the manuscript was
published in Physics Today, June 1982, page 43. In it, Fletcher claims that
he was the first to do the experiment with oil drops, was the first to
measure charges on single droplets, and may have been the first to suggest
the use of oil. According to Fletcher, he had expected to be co-author with
Millikan on the crucial first article announcing the measurement of the
electronic charge, but was talked out of this by Millikan.
Leading up to Moseley - Atomic Weights and Periodic Properties
Henry Gwyn Jeffreys Moseley was born on November 23, 1887 and would die in battle
on August 10, 1915, before he turned 28. However, as long as our civilization stands, he
will be remembered as the man who numbered the elements. That work, completed in a
six-month span during 1913 and 1914 and published in the last two papers of his life was
a tour de force of scientific accomplishment. Said Robert Milikan:
"In a research which is destined to rank as one of the dozen most brilliant
in conception, skillful in execution, and illuminating in results in the
history of science, a young man twenty-six years old threw open the
windows through which we can glimpse the sub-atomic world with a
definiteness and certainity never dreamed of before. Had the European
War had no other result than the snuffing out of this young life, that alone
would make it one of the most hideous and most irreparable crimes in
history."
A brief summary of atomic weights and periodic properties is in order. Before Moseley,
periodic tables were created on the basis of increasing atomic weight (with two
exceptions). Moseley showed that the correct ordering of the periodic table is on the basis
of the atomic number (the number of positive charges in the nucleus).
As an aside, he also showed that there are no elements lighter than hydrogen (atomic
number = 1) and that there is no possibility for elements between hydrogen and helium
(atomic number = 2). Both possibilities had been advanced, with some proposals
demanding three elements between H and He.
I. The Concept of Atomic Weight
Leucippus and Democritus (about 440 BC) are credited with the origin of the atom
concept. It was Epicurus, slightly more than 100 years later, who added weight as a
property of atoms.
The first tables of relative atomic weights were prepared by John Dalton about 1803.
There was much discussion and controvery over the next several decades concerning
atomic weights. Various authoritative chemists of the time prepared competing tables of
atomic weights with many values the same, but a significant number of differences. Some
issues were not be fully resolved until 1860, when wide-spread agreement about atomic
weight values in the chemistry community started to come together.
II. Periodic Properties of Elements
Ten elements were known from pre-historical times. Phosphorus was discovered about
1665 and from then, up to 1800, 20 more elements were discovered. There was an
explosion of element discovery starting around 1800, with 27 more elements being
discovered by the 1840s.
Starting in 1816, but not fully developed unil nearly 1830, Johann Wolfgang Döbereiner
was the first person to emphasize chemical similarities, pointing out "triads" of elements
like lithium, sodium and potassium as well as chlorine, bromine and iodine. He published
five triads as well as several "incomplete" triads.
John Alexander Reina Newlands, working after the reform of atomic weights in 1860,
was the first to proclaim a pattern for ALL elements. His tables, done in 1864 and 1865,
followed what he called the "Law of Octaves." This meant that, when ordered by atomic
weights, every eighth element showed similar chemical properties. In his early tables, he
left gaps for missing elements, but his final table of 1865 left no gaps whatsoever. Also,
he put two elements into the same position several times. Finally, he allowed for no
period longer than eight.
However, with Newlands, the "atomic number" first enters the scene. His table of 1865
shows no atomic weights and simply numbers the elements in order from 1 to 56.
Newlands' work was not favorably received. In March, 1866 he spoke on his work and
one of his listeners, a man named Carey Foster with no other claim to fame, rose to
facetiously ask if Newlands had ever attempted to classify the elements in alphabetical
order.
III. The Modern Periodic Table
The modern periodic table was developed (discovered? invented?) by Dmitri Mendeleev
during the years 1869-1871. (Some historians credit others as co-discoverers, but we will
ignore them.) Mendeleev did not use the "atomic number" that Newlands had used.
"Atomic number" remained a number without any physical meaning. It was simply the
numbering of the elements after they had been placed in order by atomic weght.
Mendeleev had periods of eight like Newlands, but he also correctly allowed for longer
periods in the transition and rare earth elements. He made a number of correct predictions
for missing elements and he had Co/Ni and Te/I in their correct chemical order -- the
reverse of the order based on increasing atomic weights. He ordered the elements on their
atomic number except for the two pairs just noted, which he put in their correct chemical
order, even though no one knew why.
It would be Moseley that finally gave the correct answer to why the elements were
reversed from a strict ordering based on atomic weights. The elements were correctly
ordered based on the atomic numbers.
Leading up to Moseley: X-Ray Spectra
Henry Gwyn-Jeffreys Moseley was born on November 23, 1887 and would die in battle
on August 10, 1915, before he turned 28. However, as long as our civilization stands, he
will be remembered as the man who numbered the elements. That work, completed in a
six-month span during 1913 and 1914 and publishd in the last two papers of his life was a
tour de force of scientific accomplishment. Said Robert Milikan:
"In a research which is destined to rank as one of the dozen most brilliant
in conception, skillful in execution, and illuminating in results in the
history of science, a young man twenty-six years old threw open the
windows through which we can glimpse the sub-atomic world with a
definiteness and certainity never dreamed of before. Had the European
War had no other result than the snuffing out of this young life, that alone
would make it one of the most hideous and most irreparable crimes in
history."
A brief summary of X-ray research is in order, since Moseley will use a regular change in
the position of lines in the X-ray spectrum of each element to assign a positive charge
(the atomic number) to the nucleus of each element.
I. The Discovery of Secondary X-Rays
Our X-ray thread starts in the evening of November 8, 1895. This is the day that Wilhelm
Conrad Röntgen discovered X-rays. He realized the importance of his discovery at once.
He stayed up all night doing experiments and even ate and slept in the laboratory for a
time. His "preliminary communication" on X-rays was turned in on December 28, 1895
and published before the end of the year. Very quickly, others began studying X-rays,
with many new discoveries being made.
For us, the next step in our story was made in 1897. It was found that, when a primary Xray beam was directed at a substance, that substance gave off secondary X-rays. (Please
realize that many other discoveries were made about X-rays. I'm just highlighting the
ones which culminate in Moseley's work.)
II. Secondary X-Rays are Characteristic of the Element
The next discovery was made by Charles G. Barkla. He found a connection between the
atomic weight of the element and its secondary X-rays. His first efforts in this area were
in 1906 (the same year Rutherford discovered alpha-particle scattering) and in 1909 he
wrote:
"It has been found that each of the elements Cr, Fe, Co, Ni, Cu, Zn, As,
Se, Ag, when subject to a suitable primary beam of X-rays, emits an
almost perfectly homogeneous beam of X-rays, the penetrating power of
which is characteristic of the element emitting it."
Barkla ordered the list of elements above by the penetrating power of the secondary
radiation with the Cr called "soft," which means not very penetrating, up to silver which
is very penetrating or "hard." Notice that the list follows the chemical order of Co then
Ni. If the list were ordered by strict atomic weights, the Ni would come first.
Since no one could yet measure the wavelengths (or frequencies) of X-rays, Barka
measured the absorbance of each secondary radiation. He did so by directing the
secondary radiation through a 0.01 cm layer of aluminum and measuring how much of
the beam was absorbed.
It turns out that the "hard" radiation (the more penetrating ones) has the shortest
wavelength (which also means the highest frequency and the highest energy). So as the
atomic weight increased (with the Co/Ni exception), the secondary X-ray became harder
and harder. "Soft" X-rays means of lower penetrating ability, so much of the secondary
beam is absorbed. (Soft means longer wavelength X-rays which also means lower
frequency and lower energy.) In the early years, elements below about aluminum could
not be studied due to the instruments not being sensitive enough to measure the X-rays
after absorption.
III. A Second X-Radiation is Found
Barkla (and his students) continued the detailed study of secondary X-radiation. In 1909,
Barkla published another paper in which he found that the supposedly homogeneous
secondary X-rays were, in fact, heterogeneous. He wrote:
"The writer has recently investigated more closely the radiations from Sn,
Sb, I (which have been recorded as elements emitting a radiation of
variable penetrating power). It has been found that these consist of a very
easy absorbed radiation and a very penetrating homogeneous radiation
superposed. The absorptions of the penetrating portions of the beams from
each element are shown in fig. I on curve B. The percentage absorptions
of the soft radiations from these elements have not yet been determined,
but they are roughly indicated on curve A in fig. 1. Though a full analysis
of the radiations from W, Pt, Au, Pb, Bi, etc., has not yet been made, there
is strong evidence that the observed radiations from these elements are
also principally homogeneous radiations characteristic of the elements
emitting them."
In 1911, Barkla wrote:
"It is seen that the radiations fall into two distinct series, here denoted by
the letters K and L*."
In the footnote indicated by the asterisk, he added:
"* Previously denoted by letters B and A. The letters K and L are,
however, preferable as it is highly probable that series of radiations both
more absorbable and more penetrating exist.
Barkla closed his paper this way:
"It has been shown that each element has its own characteristic fluorescent
line spectrum in X-rays. This is very conveniently represented as is [in?] a
spectrum of ordinary light, except that without a knowledge of the wavelength we are obliged to define the radiations by their absorption in some
standard substance. Thus we may represent the known portion of the
spectra of elements Sb, I, and Ba as in fig. 5. The lines move towards the
more penetrating end of the spectrum with an increase in the atomic
weight of the element.
It is scarcely too much to say that all the phenomena connected with the
transmission of X-rays through matter may be readily explained in terms
of a few simple laws expressed with reference to these spectra."
Barkla wrote this slightly two years before Moseley would publish his historic papers in
December 1913 and April 1914.
Moseley's Discovery - The Modern Concept of Atomic Number
Today, we know that the atomic number gives the number of protons (positive charges)
in the nucleus. This was the discovery made by Henry Gwyn-Jefferies Moseley. He
found that certain lines in the X-ray spectrum of each element moved the same amount
each time you increased the atomic number by one.
Rutherford (in 1914) described Moseley's discovery thus:
"Recently Moseley has supplied very valuable evidence that this rule
[atomic numbers changing by one from element to element] also holds for
a number of the lighter elements. By examination of the wave-length of
the characteristic X rays emitted by twelve elements varying in atomic
weight between calcium (40) and zinc (65.4), he has shown that the
variation of wave-length can be simply explained by supposing that the
charge on the nucleus increases from element to element by exactly one
unit. This holds true for cobalt and nickel, although it has long been
known that they occupy an anomalous relative position in the periodic
classification of the elements according to atomic weights."
I. Atomic Structure: 1903 - 1911
Exactly where the positive protons (and the negative electrons) were in the atom took
time to be worked out. Keep in mind that the electron (the first sub-atomic particle
discovered) was not discovered until 1897.
(1) J.J. Thomson in 1903, had electrons as negative particles with mass,
while the positive charge was spread out through the space of the atom.
(2) In 1911 Rutherford announced his atomic model: (a) a nucleus - a
dense concentration of positive charge with (b) electrons orbiting the
nucleus in an unspecified manner.
(3) In 1913, Bohr took up the question of where the negative electrons are
(in the atom) and Moseley studied where the positive charges were.
By the way, Moseley was part of Rutherford's research group -- having arrived in
Manchester just weeks before Rutherford published his great nucleus paper -- when he
started his atomic number work. Rutherford was not all that excited by Moseley wanting
to study X-rays, but the energy and enthusiasm of the younger man soon wore Rutherford
down.
[You might notice that neutrons have not been mentioned. It would not be until 1920 that
Rutherford proposed the existence of a neutral particle -- the neutron. Another of
Rutherford's students -- James Chadwick -- won the 1935 Nobel Prize for discovering the
neutron in 1932.]
Within a few months of Rutherford's nucleus paper being published, the true, physical
meaning of "atomic number" was suggested by A. van den Broek. In 1913, he wrote:
"In a previous letter to NATURE (July 20, 1911, p. 78) the hypothesis was
proposed that the atomic weight being equal to about twice the intraatomic charge, 'to each possible intra-atomic charge corresponds a
possible element,' or that (Physik. Zeitschr, xiv., 1912, p. 39), 'if all
elements be arranged in order of increasing atomic weights, the number of
each element in that series must be equal to its intra-atomic charge.' "
II. Moseley's X-Ray Spectra Work
Moseley's problem was to find a linear relationship between the atomic number and a
measureable property of the nucleus. The atomic number increased by steps of one (18,
19, 20, 21, and so on). Moseley needed some function of a nuclear property that
increased in the same pattern, that is, by one for each element in turn. He found it in the
K line of the X-ray spectra of each element. It turns out that the square root of the
frequency moves by a constant value (let's call it "one unit") for each one unit move by
the atomic number.
Why did he choose to study this area for what he needed? We can find the answer in the
work of Charles Barkla. He had demonstrated that the elements emitted characteristic Xrays, called K and L rays. These X-rays were independent of the physical or chemical
state the element was in. Someone, perhaps Barkla or Bohr or Moseley, realized that this
meant the X-rays were characteristic of the nucleus.
So Moseley set about to determine the wavelengths of the K radiation using recently
discovered techniques by the father-and-son team of W.L Bragg and W.H. Bragg. It
seems to me as I write this that Moseley was pretty confident going into this experiment
that all he needed to do was find the proper linear relationship. Getting the equipment
working so as to give reliable data was probably the most time-consuming task of the
entire research he carried out.
However, the research was carried out and Moseley determined the relationship
mentioned above. It was linear, with the frequency square root value moving up the same
amount for each one unit jump in the atomic number. Here, using Moseley's data is graph
which shows linear behavior:
About this data, Moseley himself said:
"We have here a proof that there is in the atom a fundamental quantity,
which increases by regular steps as we pass from one element to the next.
This quantity can only be the change on the central positive nucleus, of the
existence of which we already have definite proof."
The Nuclear Symbol
The nuclear symbol consists of three parts: the symbol of the element, the atomic number
of the element and the mass number of the specific isotope.
Here is an example of a nuclear symbol:
The element symbol, Li, is that for lithium.
The three, subscripted left, is the atomic number and the seven, superscripted left, is the
mass number.
Here's another:
The atomic number is:
The number of protons in the nucleus of the atom.
The mass number is:
The number of protons and neutrons in the nucleus of the atom.
Here is one last example:
The 22 is the atomic number for titanium and 48 is its mass number. The number of
neutrons is 48 minus 22 = 26.
Now, write the nuclear symbol for the chlorine isotope with 18 neutrons.
Here are two tips:
1) The element name (or symbol) uniquely determines the atomic number.
In the example just above Ti is the only element with an atomic number of
22. So, if you need the atomic number, and all you know is the specific
element, go to a periodic table and find its atomic number.
2) Suppose you are asked to write a nuclear symbol from scratch and the
teacher requires it be a realistic one. Do this:
a) Select an element, making sure it is a naturally occuring
one. This will determine its atomic number.
b) Take the element's atomic weight and round it off to the
nearest whole number. More often than not, this will be the
mass number of the most abundant stable isotope
Let's try an example. Write the nuclear symbol for silver.
Here is the answer:
Notice the mass number is rounded off from the atomic weight on the periodic table
(107.868)
How to Calculate an Average Atomic Weight
To do these problems you need some information: the exact atomic weight for each
naturally-occuring stable isotope and its percent abundance. These values can be looked
up in a standard reference book such as the "Handbook of Chemistry and Physics."
This problem can also be reversed. Study the tutorial below and then look at the problems
done in the reverse direction.
Example #1: Carbon
mass number exact weight percent abundance
12
12.000000
98.90
13
13.003355
1.10
To calculate the average atomic weight, each exact atomic weight is multiplied by its
percent abundance (expressed as a decimal). Then, add the results together and round off
to an appropriate number of significant figures.
This is the solution for carbon:
(12.000000) (0.9890) + (13.003355) (0.0110) = 12.011
Example #2: Nitrogen
mass number exact weight percent abundance
14
14.003074
99.63
15
15.000108
0.37
This is the solution for nitrogen:
(14.003074) (0.9963) + (15.000108) (0.0037) = 14.007
Example #3: Chlorine
mass
exact
percent
number
weight
abundance
Example #4: Silicon
mass
exact
percent
number
weight
abundance
35
34.968852
75.77
28
27.976927
92.23
37
36.965903
24.23
29
28.976495
4.67
30
29.973770
3.10
The answer for chlorine: 35.453
The answer for silicon: 28.086
This type of calculation can be done in reverse, where the isotopic abundances can be
calculated knowing the average atomic weight.
Practice Problems
Calculate the average atomic weight for:
1) magnesium
mass number exact weight percent abundance
24
23.985042
78.99
25
24.985837
10.00
26
25.982593
11.01
2) molybdenum
mass number exact weight percent abundance
92
91.906808
14.84
94
93.905085
9.25
95
94.905840
15.92
96
95.904678
16.68
97
96.906020
9.55
98
97.905406
24.13
100
99.907477
9.63
3) tin (this one is optional!! Suggestion: set it up as a spreadsheet, take it into class and
impress your teacher.)
mass number exact weight percent abundance
112
111.904826
0.97
114
113.902784
0.65
115
114.903348
0.36
116
115.901747
14.53
117
116.902956
7.68
118
117.901609
24.22
119
118.903310
8.58
120
119.902200
32.59
122
121.903440
4.63
124
123.905274
5.79
The answers? Look on a periodic table!! Remember that the above is the method by
which the average atomic weight for the element is computed. No one single atom of the
element has the given atomic weight because the atomic weight of the element is an
average, specifically called a "weighted" average.
The Discovery of the Electron, Proton, and Neutron
Barrie M. Peake
University of Otago, Box 56, Dunedin, New Zealand
Published in the Journal of Chemical Education, Vol. 66, No. 9
September 1989, pg. 738
Introductory courses in chemistry invariably include an account of the
historical development of our ideas on the structure of the atom. The three
fundamental particles making up an atom are introduced, naturally, but the
history of their discovery and the origins of their names, particularly for the
proton, does not appear to be well documented. The existence of a
fundamental unit of electricity was first suggested by Michael Faraday in
1834 (1) to account for his results involving the electrolysis of solutions of
aqueous acids and salts: " . . . if we adopt the atomic theory or phraseology,
then the atoms of bodies which are equivalent to each other in their ordinary
chemical action, have equal quantities of electricity associated with them."
This idea was subsequently extended by other scientists including George J.
Stoney, who first proposed in 1874 (2) the name "electrine" for the unit of
charge on a hydrogen ion; he subsequently changed this in 1891 (3) to the
name "electron". The observation of a small number of large angle
deflections of alpha particles (He2+) incident upon a gold metal foil led
Ernest Rutherford in 1911 (4) to suggest that the nucleus of an atom is very
small and positively charged. Two years later he concluded from the results
of experiments involving the scattering of alpha particles from simple gases
(5) that "the hydrogen atom has the simplest possible structure of a nucleus
with one unit charge." It appears that Rutherford was also the first
tentatively to suggest the name "proton" for this fundamental particle. This
occurred at an informal meeting around 1920 of the Physics Section of the
British Association (6): "the name proton met with general approval,
particularly as it suggests the . . . term 'protyle' given by Prout in his wellknown hypothesis that all atoms are built up of hydrogen." The term itself is
derived from the Greek protos (first), and Samuel Glasstone (7) has noted
that it had been used as far back as 1908
or earlier as a general term for a unit from which all elements were built.
From observations of the nature of the particles formed from alpha particle
scattering from N™, Rutherford was also the first to suggest in 1920 (8) that
" . . . it may be possible for an electron to combine much more closely with
the H nucleus, forming a kind of nuclear doublet. Such an atom would have
very novel properties." In discussing the classification scheme for isotopes,
William Draper Harkins first introduced in 1921 (9) the term "neutron": " . .
. a term representing one negative electron and one hydrogen nucleus". It is
interesting to note, however, that the same term had also been used by
Walther Nernst (10) at least 10 years earlier in the context of "... a
compound of positive and negative electrons . . . an electrically neutral
massless molecule". The actual observation of such a fundamental particle
had to wait until some time later when, in 1930, Bothe and Becker (11)
reported that exposure of light elements, in particular beryllium, to alpha
rays leads to a highly penetrating radiation. In 1931-1932 the Curie-Joliot
and Joliot (12) reported that exposure of hydrogen-containing material,
particularly paraffin, to this new radiation lead to the ejection of highvelocity protons. At the same time Chadwick (13) interpreted both these sets
of results in terms of radiation consisting "of particles of mass nearly equal
to that of the proton and with no net charge", which he identified with the
neutron species first postulated by Rutherford.
These details should enable teachers to complete the historical account
of the development of our present knowledge of atomic structure, which
appears to be otherwise well presented in many introductory chemistry texts.
Literature Cited
1. Faraday, M. Phil Trans. Roy. Soc. (London) 1834, 121, (Paragraph 869).
Reprinted in Experimental Researches in Electricity, Dover: New York,
1965.
2. Stoney, G. J. Phil. Mag. 1881, 11, 381.
3. Stoney, G. J. Sci. Trans. Roy. Dublin Soc. 1891, 4, 663-607.
4. Rutherford, E. Phil. Mag.1911 21,669-688.
5. Rutherford, E. Phil. Mag. 1913 26, 702-712.
6. Footnote (p 282) to Masson, O. Phil. Mag. 1921, 91, 281-285
7. Glasstone, S. Sourcebook on Atomic Energy; Macmlllian: London, 1960;
footnote
p 42.
8. Rutherford, E Proc. Roy. Soc. 1920, 97, 374-400.
9. Harkins, W. D. J. Am. Chem. Soc. 1921, 43, 1038-1060
10. Nernst, W. Theoretical Chemistry, 3rd ed., Marmillan: London, 1911: p
396.
11. Bothe, W.; Becker, H. Z. Phys. 1930, 67, 289-306.
12. Curie-Joliot, I.; Joliot, F. Compt. Rend. 1931, 143, 1412-1414, 1416
1417: 1932,
144, 273-276, 708-711, 876-877.
13. Chadwick, J. Nature l932, 129, 312; Proc.Roy.Soc. 1932,136, 692-708:
1933, 142,
1-25
Chronology of Discoveries in Atomic Structure
c. 430 B.C. Leucippus and his pupil, Democritus (c. 460-371 B.C.) of Adbera develop the
materialistic philosophical concept of atoms (from atomos, atomos, “uncuttable”). Matter
did not form a continuium, but consisted of atoms which are 1) solid, 2) impenetrably
hard
(or cannot be divided), 3) eternal, 4) homogeneous and identical in substance, and 5)
there
are a finite number of kinds. Motion is inherent in the atoms (which move in a void) and
they differ in their geometrical and mechanical properties. From these qualities of size,
shape, and motion our world appears by collision and conglomeration, just as a book
arises
from different shaped letters and different ordering of the letters.
Atomism did enjoy some success among followers of Epicuris (c. 341-270 B.C.), but it
was
soon pushed aside (especially by Plato and Aristotle) and did not receive serious
consideration until the seventeenth century. Both Aristotle and Plato rejected the idea of a
vacuum or void and the concept of self-moving bodies. However, Aristotle did admit to
there
being a practical lower limit to a substance being subdivided. He called this minima
naturalis.
Examples of Democritus’ atoms
1417 Rediscovery of the poem De Rerum Natura (On The Nature of Things), written by
Lucretius (c. 95 - 55 B.C.). This long poem was based on atomistic teachings and
explained
much of its philosophy. To this day, very little of Lecuippus’ and Democritus’ writings
remain.
1660 Pierre Gassendi succeeds in freeing atomic theory from godlessness. Because of its
athestic basis as compared to Christianized Aristotle, atomism had only appealed to a few
radical philosophers. The general philosophical problem remained, how can it be proved
that matter is particulate in nature. The general notion of explaining phenomena in terms
of particles begins to arise through the efforts of Issac Newton and Robert Boyle.
1666 Boyle, although not a chemical atomist, did think of matter in finite, ultimate
particles too small to be individually perceptible to the senses.
1687 Newton shows that Boyle’s Law (pressure of a gas is inversely proportional to its
volume) follows if a gas is made up of mutually repulsive particles, with the forces
inversely proportional to their distances apart. This assumption is incorrect, but it is
historically important for being among the first uses of atoms or particles in an
explanation. Dalton thought Newton had proved the gas molecules are mutually
repelling,
but was still able to develop much that was correct in his atomic theory.
1704 Newton wrote, “It seems probable to me that God formed matter in solid, massy,
hard, impenetrable, movable particles.... God is able to create particles of matter of
several sizes and figures....”
1738 Daniel Bernoulli uses the atom idea to correctly account for Boyle’s Law.
1803 John Dalton postulates the existence of atoms and publishes the first table of atomic
weights. This hypothesis enables him to explain many features of chemistry on a simple
basis. Dalton today is known as the “Father of Modern Atomic Theory.” Modern
scholarship has identified 4 postulates implicit in Dalton’s work. They are: 1) Elements
are made up of minute, discrete, indivisible, and indestructible particles called atoms.
These atoms maintain their identity through all physical and chemical changes. 2) Atoms
of the same element have the same properties. Atoms of different elements have different
properties. 3) Atoms of the same element can unite in more than one ratio with another
element to form more than one compound. 4) Chemical combination between two or
more atoms occur in simple, numerical ratios (i.e., 1 to 2; 2 to 3; ec.).
While it becomes clear that atoms must exist, since Dalton’s results are explainable only
on an atomic basis, there is still no knowledge about what the structure of an atom is or
how they can connect (or bond) together. To Dalton, atoms were hard, featureless spheres
which must exist, but he had no knowledge of their inner structure.
1833 Michael Faraday discovers quantitative laws of electrochemical deposition. He
introduces the terms electrode, cathode, anode, ion, anion, cation, electrolyte. He
establishes that a definite quantity of electricity is associated with each atom of matter.
The number of atoms that chemically can react is related to the number of electrons
available in the system. In other words, Faraday measured and quantized electricity; he
identified a special, integral electron-to-atom relationship. He said, “...if we adopt the
atomic theory or phraseology, then the atoms of bodies which are equivalent to each
other in their ordinary chemical action, have equal quantities of electricity associated
with them.”
1838 Faraday studies electric discharges in a vacuum and discovers the Faraday dark
space near the cathode.
1886 Eugen Goldstein observes that a cathode-ray tube produces, in addition to the
cathode ray, radiation that travels in the opposite direction - away from the anode; these
rays are called canal rays because of holes (canals) bored in the cathode; later these will
be found to be ions that have had electrons stripped in producing the cathode ray.
1855 German inventor Heinrich Geissler develops mercury pump - produces first good
vacuum tubes, these tubes, as modified by Sir William Crookes, become the first to
produce cathode rays, leading to the discovery of the electron.
1858 Julius Plucker shows that cathode rays bend under the influence of a magnet
suggesting that they are connected with in some way; this leads in 1897 to discovery that
cathode rays are composed of electrons.
1865 H. Sprengel improves the Geissler vacuum pump. Pluecker uses Geissler tubes to
show that at lower pressure, the Faraday dark space grows larger. He also finds that there
is an extended glow on the walls of the tube and that this glow is affected by an external
magnetic field.
1869 J.W. Hittorf finds that a solid body put in front of the cathode cuts off the glow
from the
walls of the tube. Establishes that “rays” from the cathode travel in straight lines.
1871 C.F. Varley is first to publish suggestion that cathode rays are composed of
particles. Crookes proposes that they are molecules that have picked up a negative charge
from the cathode and are repelled by it.
1874 George Johnstone Stoney estimates the charge of the then unknown electron to be
about 10-20 coulomb, close to the modern value of 1.6021892 x 10-19 coulomb. (He
used the Faraday constant (total electric charge per mole of univalent atoms) divided by
Avogadro’s Number. James Clerk Maxwell had recognized this method soon after
Faraday had published, but he did not accept the idea that electricity is composed of
particles.) Stoney also proposes the name “electrine” for the unit of charge on a hydrogen
ion. In 1891, he changes the name to “electron.”
1876 Eugen Goldstein shows that the radiation in a vacuum tube produced when an
electric current is forced through the tube starts at the cathode; Goldstein introduces the
term cathode ray to describe the light emitted.
1881 Herman Ludwig von Helmholtz shows that the electrical charges in atoms are
divided into definite integral portions, suggesting the idea that there is a smallest unit of
electricity.
1883 H. Hertz shows that cathode rays are not deflected by electrically charged metal
plates, which would seem to indicate that cathode rays cannot be charged particles.
1890 A. Schuster calculates the ratio of charge to mass of the particles making up cathode
rays (today known as electrons) by measuring the magnetic deflection of cathode rays.
1892 Heinrich Hertz who has concluded (incorrectly) that cathode rays must be some
form of wave, shows that the rays can penetrate thin foils of metal, which he takes to
support the wave hypothesis. Philipp von Lenard develops a cathode-ray tube with a thin
aluminum window that permits the rays to escape, allow in the rays to be studied in the
open air.
1894 Joseph John (J.J.) Thomson announces that he has found that the velocity of
cathode
rays is much lower than that of light. Stoney introduces the term electron.
1895 Jean-Baptiste Perrin shows that cathode rays deposit a negative electric charge
where they impact, refuting Hertz’s wave concept and showing that the cathode ray are
particles.
1896 Pieter P. Zeeman discovers that spectral lines of gases placed in a magnetic field are
split, a phenomenon call the Zeeman effect; H.A. Lorentz explains this effect by
assuming that light is produced by the motion of charged particles in the atom. Hendrik
Antoon Lorentz uses Zeeman’s observations of the behavior of light in magnetic field to
calculate the charge to mass ratio of the electron in an atom, a year before electrons are
discovered and 15 years before it is known that electron are constituents of atoms.
1897 Walter Kaufmann determines the ratio of the charge to mass for cathode rays in
April, about the same time Thomson does, but Kaufmann fails to consider that the rays
might be subatomic particles. Thomson discovers the electron, the first known particle
that is smaller than the atom, in part because he has better vacuum pumps than were
previously available; he, and independently, Emil Weichart, determine the ratio of charge
to mass of the particles by deflecting them by electric and magnetic fields. Thomson’s
work can be divided into three components: 1) he improved J. Perrin’s method of
collecting charge inside the vacuum tube, showing that a charge was collected only when
a magnetic field was used to bend the rays into a path leading to the collector, 2) he
showed that the cathode rays are deflected by an electric field. He explained Hertz’s
results (see 1883 above) by hypothesizing that gas remaining in the tube was ionized, the
ions collect on the plate and neutralize the charge on the plates. Using better vacuum
pumps avoided this problem.
3) he was able to obtain a good value for the charge to mass ratio in two independent
ways – from the temperature rise on the charge collector and by balancing the magnetic
and electric deflections of the cathode ray beam.
It is important to underscore the fact that the experiment did yield the SAME charge-to
mass ratio (e/m) for every substance tested. Magnets and electrical fields can be used to
deflect the cathode ray. The deflection is related to both charge and mass of a particle.
Knowing the strengths of the magnetic and electrical fields used allowed Thomson to
calculate the e/m ratio, but NOT ‘e’ or ‘m’ alone.
Wilhelm Wein deflects canal rays with magnetic and electrical fields. From the direction
and magnitude of the deflection, he concludes that they are positively charged particles
with charge-to-mass ratios at least a thousand times greater than Thomson’s particles. In
fact, the ratios are comparable to e/m ratios of electrically charges atoms, as measured in
electrolysis in solutions. He concludes that canal rays are atoms or molecules of gas that
have had electrons knocked out of them and thus are attracted to the negative cathode.
Most hit the cathode, but some slip through the holes where they can be studied.
Another observation was that most substances tested would release more than one
electron as the voltage on the CRT increased. Helium released 1 or 2, lithium up to 3,
beryllium up to 4, and so on. Hydrogen, as the exception, never released more than one.
Therefore, it was assumed to contain one electron and one proton (the fundamental
positive charge)
Since all substances contain electrons, if they are neutral, they must therefore contain
positive charges as well. Thomson, in his study of positive canal rays (in the late 1980’s
and early 1900’s), found its e/m value to differ for every substance tested. In that sense,
the canal ray tube represented the first mass spectrometer, a device that separates
particles of varying mass by differences in deflection when acted upon by a particular
magnetic field.
Thomson's experiment with canal rays revealed at least two kinds of neon atoms, one
with an atomic mass of 20, the other 22. The development work on the mass spectrometer
was continued by Francis Ashton, who later won a Nobel Prize for his work. The mass
spectrometer is now a fundamental piece of equipment for college-level and above.
1899 Thomson, using Charles Wilson’s condensation chamber, proves that cathode
particles carry the same amount of charge as hydrogen ions in electrolysis; he measures
the charge of the electron and thus completes his discovery of the electron; he also
recognizes ionization to be a splitting of atoms and that particles emitted by the
photoelectric effect have the same charge to mass ratio as cathode rays.
1902 Lord Kelvin suggests that the positive electricity in an atom is spread in a diffuse,
homogeneous manner through out the entire spherical volume. The negative electrons,
which are individual particles are embedded in this positive electricity, which is seen as a
jelly-like material. The amount of positive electricity is enough to counterbalance the
negative, making the atom neutral in charge.
1903-1904 Thomson investigates this model more closely and makes a series of
calculations concerning its stability. It comes to be known as the Thomson model. Note,
however, that no experiment gave rise to this model, it arose out of a need to satisfy
mechanical and electrical stability. It was quickly seen that an analysis of the scattering
of alpha particles by a thin piece of metal could serve to test the model for accuracy.
Please note also that the sphere of positive charges are not protons. The electrons are seen
as hard objects embedded in this cloud of positive charge. Lenard suggests that the
positive and negative charges are grouped in pairs.
Electrons spherical cloud of positive charge (+8 in amount)
1904 “Kinetics of a System of Particles Illustrating the Line and Band Spectrum and the
Phenomena of Radioactivity” by Hantaro Nagaoka includes his “Saturnian model” of the
atom, in which a positive nucleus is surrounded by a ring of thousands of electrons. This
model was rejected by Thomson when it was shown to be unstable. Thomson’s “On the
Structure of the Atom” proposes the “plum-pudding model” of the atom in which the
electrons are embedded in a sphere of diffused positive charge. (The series of
publications by Thomson’s group expounding on the model extended over the period
1903-1904)
1906 Ernest Rutherford studies scattering of alpha particles as they pass through mica.
Later he uses gold foil. Scattering is a classic technique that is still used in science today.
Arthur Compton uses the scattering of electrons and high energy radiation in the 1920s
and Robert Hofstadter uses it in the 1950s to study the fine structure of the nuclei and
nucleons. J.J. Thomson is awarded the Nobel Prize.
1907 Hans Geiger begins a program of research on the scattering of alpha particles as
they
pass through thin metal foils. Alpha particle scattering had been discovered in 1903 by
Rutherford and had initially been seen as a problem plaguing his research, so Geiger's
measurements would help to remove its influence in calculations.
1908 Geiger reports that the number of scattered particles decreases rapidly with
increased scattering angle and that no alpha particles were observed to be scattered by
more than a few degrees. Charles G. Barkla discovers that each element has a
characteristic X-ray, produced by scattering of X-ray beams; this is the key discovery that
eventually leads to the concept of atomic number.
1909 Ernest Marsden, under the direction of Geiger and Rutherford, determines that some
alpha particles bounce back from a thin gold foil. Many years later Rutherford recalls the
events: One day, Geiger came to me and said, “Don't you think that young Marsden,
whom I am training in radioactive methods, ought to begin a small research?”
Now I had thought that too, so I said, “Why not let him see if any alpha particles
can be scattered through a large angle?” I may tell you in confidence that I did not
believe that there would be, since we knew that the alpha particle was a very fast
massive particle, with a great deal of energy, and you could show that if the
scattering was due to the accumulated effect of a number of small scatterings the
chance of an alpha particle being scattered backwards was very small. Then I
remember two or three days later Geiger coming to me in great excitement and
saying, “We have been able to get some of the alpha particles coming
backwards…” It was quite the most incredible event that has ever happened to me
in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of
tissue paper and it came back and hit you.
Geiger found that the most probable scattering angle is 0.87 degrees, but about 1 alpha
particle in 8,000 is scattered through an angle greater than 90 degrees. This result is not in
accord with Thomson’s model of 1904. As the alpha particle approached the center of the
atom, it would be in a region of average zero electric charge and could not be deflected.
The same would apply if it went too far from the center.
1911 Rutherford, presents his theory of the atom, consisting of a charged, small, dense
nucleus, to the Manchester Literary and Philsophical society on March 7. Hans Geiger
recalled many years later how he found out Rutherford's theory:
“…one day, Rutherford, obviously in the best of spirits, came into my room and
told me that he now knew what the atom looked like and how to explain the large
deflections of alpha particles.”
Only an abstract of that early March talk survives, but later in the year he published a
long article explaining his ideas. Interestingly, the alpha particle deflections observed
would have been the same if the nucleus had been either positively of negatively charged.
In fact, Rutherford wrote, “The main deductions from the theory are independent of
whether the central charge is supposed to be positive or negative.” He finally used the
velocities of alpha particle emission by radioactive elements to eliminate the negative
charge. (Replication of the Rutherford experiment in a world of antimatter would yield
the same results found in our world of matter.) One problem of the model had to do with
the orbiting electrons. James Clerk Maxwell had shown that moving electric charges (i.e.
electrons in this case) give off energy and should spiral into the nucleus. They do not.
Hence, how to explain this. Geiger and Marsden start a program of carefully measuring
the fraction of alpha particles scattered by various angles. In 1913 they report that their
experimental results are in good agreement with Rutherford’s calculations. Thus they
establish that Rutherford’s picture of an atomic nucleus surrounded by electrons is
correct. Rutherford also concludes that nuclear charge is about half the atomic weight.
Charles Barkla reaches the same conclusion from x-ray scattering experiments.
The Rutherford Model Rutherford's Model of a thin gold foil Thomson's Model might be
might look like this: pictured this way The nucleus dots are way, way out of scale (as in
way, way too big) in the Rutherford Model.
The nucleus was so tiny (10-12 cm in diameter), that the atom (10-8 cm in diameter) was
99.99% empty space.
In the Rutherford Model, the alpha particles mostly went through empty space, but once
in a while one came close to the nucleus (Rutheford's "charge concentration") and was
deflected strongly (more than 90°). The Thomson Model has no such possibility because
the charges were equally spread throughout the atom.
1912 Neils Bohr visits Rutherford and seizes on the problem of explaining the behavior
of the electrons in their orbits around the nucleus. He also identifies atomic number (up to
this time simply the numerical order of the elements in the periodic table) with the
nuclear charge.
1913 Antonius van der Broeck, notes that atomic mass and the number of electrons in an
atom are independent. He also suggests, independent of Bohr, that the nuclear charge is
exactly equal to the “atomic number” used to order the elements in the periodic table.
Bohr publishes a formula which gives the length of the radiation (usually X-rays) given
off when an electron enters one of the innermost orbits of the atom in terms of the
electrical charge on the nucleus. Henry Gwyn-Jeffries Moseley, deduces the place of
elements in the periodic table from their X-ray spectra and formulates his law of numbers
of electrons; an element has the same number of electrons as its atomic number. Before
this time, the elements had been put in order (with a few exceptions) by increasing atomic
mass. Moseley comfirms Bohr’s prediction and associates this order with an increase in
the nuclear charge that increases in whole number multiples of the electron charge. (The
only difference being the nuclear charge is positive and the charge on the electron
negative.)
1913 Rutherford, from continued scattering experiments, concludes that “the hydrogen
atom has the simplest possible structure of a nucleus with one unit charge.”
1914 Robert A. Millikan measures the charge on the electron directly. His work shows
that the electric charge always comes in integer multiples of 1.592 x 10-19 coulomb. This
is less than 1% from the currently accepted value of 1.60217733(49) x 10-19 coulomb.
The 49 refers to the plus/minus error in the last two digits (the 33).
1915 Moseley is killed in battle.
1919 Rutherford carries out the first artifical nuclear reaction. In the decay products are
nuclei of hydrogen (what we now call protons). He recognizes this particle as being truly
elementary, on a par with the electron.
1920 Rutherford proposes the name “proton” for the fundamental particle which makes
up the hydrogen nucleus. The word proton had been used from about 1908 as a general
term for a building block from which all elements are built. He also proposes the
existence of the neutron, an uncharged particle that is part of the nucleus. He sees it as a
combination of an electron and a proton. He says, “... it may be possible for an electron to
combine much more closely with the H nucleus, forming a kind of nuclear doublet. Such
an atom would have very novel properties.” Today we see it as a particle in and of itself,
not a combination (although, see quarks). He also proposed the name proton for the
nucleus of the hydrogen atom. Interestingly, he also proposes the existence of a hydrogen
isotope with mass two. Today it is called deuterium.
1921 William Draper Harkins introduces the term neutron: “... a term representing one
negative electron and one hydrogen nucleus.”
1932 James Chadwick discovers the neutron. Harold Urey discovers deuterium.
1950 - 1957 Robert Hofstadter uses the scattering of electrons by the nucleus to study the
detailed structure of the nucleus, protons, and neutrons.
1964 Richard Feynman and George Zweig independently propose the parton model for
the structure of protons and neutrons. Partons come to be known as quarks.
And study continues to this day.
J.J. Thomson Discovers the Electron
1. Cathode ray bends toward positive electrical pole Why? Ray is negative in charge.
2. Cathode ray bends under magnet. Why? Electricity and magnetism are two forms of
the same force. A moving electrical charge creates a magnetic field. (This point is
developed in physics, not chemistry.) The two magnetic fields interact and since the lines
of force are curved, the beam bends.
3. Evidence that cathode rays were particles, not electromagnetic radiation
a. they had a charge, i.e. deflected by electric or magnetic fields
b. had a detectable mass
c. traveled at less than the speed of light
4. What did Thomson measure?
a. velocity of beam
b. displacement of beam
c. electric field strength
d. magnetic field strength
e. amount of time spent in fields
5. When he solved his equations, he determined the charge-to-mass ratio (symbol e/m).
He could not determine e or m independently with a cathode ray. The best determination
of e was done by Robert A. Millikan in 1914 in the oil-drop experiment (p. 227 in the
text). A clear and lucid explanation of the equations Thomson used and his reasoning is
found in The Discovery of Subatomic Particles (1983). Steven Weinberg. W.H Freeman:
New York.
6. Thomson determined that the e/m ratio was the same for every combination of gas
present in the tube and metal used as cathode and anode. In other words, suppose you
measured the e/m ratio found in every possible combination of these gases: hydrogen,
chlorine, nitrogen, neon and these metal electrodes: gold, silver, copper, platnium. The
value was ALWAYS the same.
7. Why do We Remember Thomson as the Discoverer of the Electron?
To put it into his own words,
“…we have in the cathode rays matter in a new state, a state in which the subdivision of
matter is carried very much farther than in the ordinary gaseous state: a state in which all
matter–that is, matter derived from different sources such as hydrogen, oxygen, etc.–is of
one and the same kind; this matter being the substance from which the chemical elements
are built up.”
He had leaped to the conclusion that the particles in the cathode ray (which we now call
electrons) were a fundamental part of all matter. This was reaching quite far beyond what
he had actually discovered. As he was to recall much later,
”At first there were very few who believed in the existence of these bodies smaller than
atoms. I was even told long afterwards by a distinguished physicist who had been present
at my [1897] lecture at the Royal Institution that he thought I had been ‘pulling their
legs.’ ”
Other people had measured the e/m ratio or suggested that the cathode rays were
composed
of particles, but Thomson was the first to say that the cathode ray was a building block of
the atom. It was a risky thing, but he was proved right and for his courage he is
remembered as the discoverer of the electron.
J.J. Thomson Discovers the Proton
Remember that J.J. Thomson announced the discovery of the electron in
1897.
This work culminated in about 1902.
1. The Production of Canal Rays
a. Cathode rays are produced, which travel cathode to anode.
direction of cathode ray cathode (negative) anode (postive)
b. Cathode rays (made of electrons) hit residual gas molecules between
cathode and anode.
c. Electrons are removed from the gas molecules by the impact. This makes
the gas molecules positive in charge.
d. The positively charged gas molecules are attracted to the negative
cathode. positive molecules move to cathode cathode (negative) anode
(positive)
e. Some slip through the holes in the cathode (the canals) and are studied.
Note that the cathode and anode are placed close together.
cathode anode the arrow shows the direction of the canal ray.
2. Both types of rays are produced in the two types of tube. The location and
design of the cathode and anode determine if cathode rays or canal rays are
studied.
3. Thomson determined the e/m ratio of the canal rays. He found that it
varied depending on the type of gas that was present in the tube. There were
also subtle variations within each value that led to the concept of isotopes. In
other words, when hydrogen, chlorine, nitrogen, or neon was used, each had
an e/m value different from the others. In the case of neon, as Thomson's
equipment and technique became better, he noticed that neon gave two spots
(very close to each other) at the same time. In other words, the same sample
of neon was producing two slightly different e/m ratios.
4. The largest value obtained in these experiments was the e/m ratio for
hydrogen. In 1898, W. Wein determined the e/m ratio for hydrogen in canal
rays to be about 104. This was consistent with the values from electrolysis.
5. The e/m ratio for hydrogen was known from electrolysis experiments
dating to the 1830's. It was about 105. Hydrogen had the largest e/m ratio
when compared to other elements, because it was the lightest element.
(Electrolysis - put anode and cathode in solution of chemicals. Turn on
current. A chemical reaction takes place. From 1830's, it had been
established by Faraday that a given amount of current (e) would produce a
given mass of chemical (m).)
6. Note that Thomson knew the e/m ratio for the electron to be 1.76 x 10 8.
(This is expressed in the modern way. I think Thomson discussed it as the
m/e ratio.) This meant that the electron must be much lighter than the
hydrogen atom. A critical assumption (see #8 below) here was that hydrogen
and the electron both had the same absolute amount of electrical charge,
with hydrogen being positive and the electron negative.
7. Also, canal rays differed from cathode rays in another important respect.
For example, helium had two possible e/m ratios, but not at the same time.
At low voltage it had one ratio and at high voltage it had another ratio. It
never showed both e/m ratios at the same time and it never switched values.
It was always the same value at the low setting and always some other value
at the higher setting. This was explained as follows. At low voltage one
electron was released. At higher voltage, two electrons were released.
8. Hydrogen was the exception. It NEVER released more than one at any
given voltage and therefore had ONLY one e/m ratio. This was explained as
hydrogen having one electron, with one negative charge. And it had one
positive charge (eventually called the proton).
9. Today the electron is known to be 1837 (p. 228 of Heath text) times
lighter than the proton.
The Proton
Carl E. Moore and Bruno Jaselskis
Loyola University, Chicago, IL 60626
Alfred von Smolinski
University of Illinois at Chicago, Chicago, IL 60637
Published in the Journal of Chemical Education, Vol. 62, No. 10
October 1985, pg. 859-860
The interesting anectodotal material describing the series of events
which led to the recognition of the proton as a fundamental particle of nature
seems worthy of a more prominent place in the teaching of physics and
chemistry than currently given it. Modern texts, at best, give only a few of
the bare bones that litter the trail of this fundamental particle. Since
essentially no uncombined hydrogen nuclei exist in electron donor solvents
(1), such as water, it is not surprising that the proton was first perceived in
the plasmas of the gas phase reactions which occur at low pressures in
electrical discharges.
The roots of this discovery can be traced back to the early works of
Michael Faraday (2), who appears to have done the first experiments (3,4)
on continuous electrical discharges through rarefied gases. A contemporary
of Faraday, William Robert Grove, an experimenter par excellence, carried
out a series of experiments on electrical discharges through gases, which he
described in the Philosophical Transactions of the Royal Society in 1852 (5).
As a consequence of these experiments, Grove reasoned that anions and
cations are formed when an electric discharge proceeds through a tube
containing a mixture of rarefied hydrogen and oxygen. To explain the results
of his experiments, he applied the von Grotthuss Model of electrical
conduction by electrolytes. This was the popular model until the
advent of Arrhenius theory. Grove was familiar with this theory and had, in
fact, pointed out an inadequacy in the theory when it was applied to his gas
battery.
Faraday had introduced the terms anion and cation in 1834. Thus,
Grove was thinking in terms of hydrogen as a cation as early as 1852. It
seems unlikely that Grove could have had a correct vision of the mechanism
of proton production, but one should not rule out the tantalizing possibility
that his visions were decades ahead of his time. We offer an excerpt from
Grove's paper which we believe is the first mention of the hydrogen cation
in the gas phase (5).
As may be gathered from my opening remarks, the experiments above
detailed appear to me to furnish a previously deficient link in the chain
of analogy connecting dielectric induction with electrolysis. The only
satisfactory rationale which I can present to my own mind of these
phenomena is the following. The discharges being interrupted . . ., the
gaseous medium is polarized anterior to [prior to] each discharge, and
polarized not merely physically, as is generally admitted, but chemically,
the oxygen or anion being determined to [appearing at] the positive
terminal or anode, and the hydrogen or cation being determined to
[appearing atl the negative terminal or cathode; at the instant preceding
discharge there would then be a molecule or superficial layer of oxygen
or of electro-negative molecules in contact with the anode, and a similar
layer of hydrogen or of electropositive molecules in contact with the
cathode, in other words, the electrodes in gas would be polarized as the
electrodes in liquid are.
William Robert Grove, who held a professorship in the London
Institution, has received only limited attention in the literature. His
interesting mix of interests, law and natural philosophy, placed Grove in a
position to play pivotal roles in both the experimental and structural sides of
the science of his time. His legal talents and training allowed him via shrewd
committee service to exercise a powerful behindthe- scenes influence in the
restructuring of the Royal Society, thus making it into a formidable
organization of scientists. His talents as a scientist brought about the
invention of the first fuel cell, which was based on hydrogen and oxygen
electrodes, and the construction of one of the most popular voltaic cells of
his time.
A contemporary of Faraday and Grove and the vice president of the
Royal Society, John P. Gassiot, in his Bakerian Lecture of 1858 reported
deflections of the electrical discharges in rarefied gases by both magnetic
and electrostatic means, but another contemporary and great experimenter in
discharge-tube phenomena, J. Plücker in Germany, is credited with the
discovery of cathode rays. However, it was Eugen Goldstein some decades
later who gave these rays the name Kathodenstrahlen (cathode rays), and it
was W. Hittorf, a student of Plücker, who first noted that objects in the path
of cathode rays cast shadows (2). Research in this area of discharge tube
phenomena was greatly accelerated by the development of the high-voltage
transformer by Ruhmkorff (5) and the large-scale capacitor by Despretz (5).
At this stage in the development of the understanding of gaseous
discharge phenomena, a dominant intellect entered physics from the field of
physiology. This man, Herman Ludwig Ferdinand von Helmholtz, had
recently completed his great treatises on physiology and was looking for
new fields to conquer. With the death of Gustav Magnus the chair in physics
at Berlin was vacated. Helmholtz assumed the chair with the avowed
purpose of bringing order into what he characterized as a pathless wilderness
of competing theories and mathematical formulas. While his name is not
directly associated with the proton, he sponsored both Eugen Goldstein
(7,8) and Wilhelm Wien (9), who did the definitive studies on this
fundamental particle. In fact, von Helmholtz's name appears on both the
1876 and 1886 papers of Goldstein. Thus he seems to have cast a large and
perhaps benevolent shadow over this development.
Eugen Goldstein, who is credited with the discovery of canal rays, after a
year (1869-1870) at Breslau joined von Helmholtz at Berlin, where he
received the doctorate in 1881. We call attention to two papers by Goldstein
in which he described the use of perforated cathodes. The first of these was
published in 1876 (7) and the second 10 years later, 1886 (8). Both appeared
in the Monthly Report of the Royal Prussian Academy of Science at Berlin,
and both were sponsored by von Helmholtz. The second is the oft quoted
paper on Kanalstrahlen (canal rays). One can only wonder if the great
intellect of von Helmholtz made a direct input into this research.
Goldstein observed that in a tube fitted with a perforated cathode
containing a rarefied gas a sheaf of light rays (canal rays) came through each
perforation in a direction opposite the path of the cathode rays (electrons).
The relatively weak magnetic fields that he employed did not give a
discernible deflection of these light rays. However, the identical magnetic
fields strongly deflected the cathode rays. His conclusion was that for the
canal rays observed he was dealing with a phenomenon which he could not
explain. These interesting observations of Goldstein lay buried in the
monthly reports of the Berlin Academy for nearly 12 years. During the latter
part of this 12-year period, Röntgen discovered Röntgen Strahlen (X-rays)
and published his first two communications. They appeared in the Monthly
Reports of the Würzburger Physics and Medicine Society, a publication of
limited circulation. Georg Wiedemann, a very perceptive editor, sought the
permission of Röntgen to republish these two seminal papers in his widely
distributed and prestigious Annalen. Röntgen, at first, refused permission,
but after publishing a third paper in the monthly reports of the Berlin
Academy he allowed Wiedemann to republish all three papers (10). In the
64th volume of the Annalen, Wiedemann followed Röntgen's three papers
with a fourth: namely, the 1866 paper of Goldstein.
It is worth noting that Goldstein's work attracted little attention in the
circle of German physicists (10) largely because they considered his
research too descriptive. In addition, Goldstein's work seemed to point to an
explanation based on the presence of particulate matter. At that time the best
opinion in the German circle on the origin of gaseous tube discharge
phenomena was based on an electromagnetic concept (10).
We are principally interested in the proton, a canal ray. However, the
name canal ray came about from a general phenomenon. Goldstein, while
experimenting with a number of gases, not hydrogen alone, noted that these
strange rays changed their color from gas to gas (8). He suggested calling
them canal rays until such time that someone selected a suitable name. His
provisional name became the accepted name.
It remained for Wilhelm Wien, who authored his papers as Willy
Wien, to recognize that canal rays were positively charged particles. He
noted that one could not visually distinguish them from weak cathode rays,
but even with a weak horseshoe magnet that the cathode rays could be
deflected and the canal rays were not noticeably deflected (9). Finally he
noted that electrostatic deflection served as a good means of identifying the
canal rays, for the canal rays were deflected to the negative pole of the
electrostatic device. He also specifically stated that the positive electricity
carried by the canal rays was an identifying characteristic of the rays.
Wien designed deflection equipment using potentials up to 30,000 volts and
determined e/m ratios for the proton where e is the charge and m is the mass
of the particle. His results agree rather well with results obtained by later
investigators.
From his measurements on discharges through hydrogen he said that
one is easily led to the opinion that canal rays were the hydrogen ions
themselves. Thus, it appears that to Wien must go the credit for the
following: recognition that the canal rays produced in electrical discharges
in low pressure hydrogen gas are positively charged particles. recognition
that these rays contain hydrogen ions, and the first e/m measurements of the
proton.
It is not clear just how the term proton (from the Greek protos, first)
became associated with the positively charged hydrogen atom. The best
source regarding this choice of a name seems to us to be found in a footnote
by E. Rutherford that is appended to a paper by O. Masson (11). However,
this interesting footnote does not give a definitive answer as to whom the
choice of the term should be attributed.
In May 1907, J.J. Thomson followed up on Wien's measurements with
a paper entitled "On Rays of Positive Electricity" (12). In these experiments
and e/m measurements he used improved apparatus and greater experimental
sophistication and observed both the proton and what appears to be the
hydrogen molecule cation [H2+]. The reader is referred to J.J. Thomson's
historic treatise "Conduction of Electricity Through Gases" (13) for further
information on the instrumentation, experimental method, mathematical
treatment, and additional points of history in the recognition of the proton.
Literature Cited
(1) Bell,R.P.,"Acids andBases—Their Quantitative Behaviour,"JohnWiley
and Sons,
Inc., New York, 1952.
(2) Hittorf, W., "Über die Elekricitätsleitung der Gase," Ann. Phys. Chem.,
CXXXVI, 1, (1869).
.
(3) Faraday, Michael, "Experimental Researches No.'s 1542, 1543, and
1544, 1838.
(4) Faraday, Michael, "Experimental Researches In Electrochemstry," No.
576 of
Everyman's Library, E.P. Dutton and Co., Inc., New York, 1914.
(5) Grove, W.R., "VII. On the Electro-Chemical Polarity of Gases," Phil.
Trans. Roy.
Soc., 142, (I) 87 (1852).
(6) Arrhenius, Svante, "Text-Book of Electrochemistry," Longmans, Green
and Co.,
New York, 1902.
(7) Goldstein, Eugen, "Vorläufige Mittheilungen über electrishe
Entladungen in
Verdünnten Gasen," Berlin Akd. Monatsber., 279 (1876).
(8) Goldstein, E., "Über eine noch nicht untersuchte Strahlungsform an der
Kathodeinducirter Entladungen," Berlin Akd. Monatsber., II, 691 (1886).
(9) Wien, W., "2. Untersuchungen über die electrische Entladung in
verdünnten
Gasen," Ann. der Physik 8, 244 (1902).
(10) Ruckhardt, E., "Zur Entdeclcung der Kanalstrahlen vor fünfzig Jahren,"
Naturwissenschaften, 30, 465 (1936).
(11) Glasstone, Samuel, "Source Book on Atomic Energy," D. van Nostrand
Company, Inc., New York, 1950.
(12) Thomson, J.J., "XLVII. On Rays of Positive Electricity," Phil. Mag.,
13, 561
(1907).
(13) Thomson, J.J., and Thomson, G.P., "Conduction of Electricity Through
Gases,"
3rd. ed., Cambridge University Press, 1928.