Download The Atom`s Ancestry s Ancestry

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The Atom’
s Ancestry
Atom’s
What in the world is an atom? Or, more appropriately; what in the world is not an atom?
Air, water, earth, people, robots—everything is made up of atoms.
As early as 500 B.C. the Greeks speculated that matter can be split into smaller and
smaller bite, but they expected a limit, beyond which it could not be further subdivided.
Etymology of Atom
We come to know from Aristotle that the founder of the atomic theory was Democritus and
Leucippus. The word ‘atom’ is derived from atomos, ‘a’ = not and tomos = a cut; thereby
meaning ‘indivisible’. The concept of atom that Western scientists accepted in broad outline
from 1600s until 1900 originated with Greek philosophers in the 5th century. The atom was
described as being hard; having a form, size, and weight and being in ceaseless motion. This
speculation was replaced slowly by scientific theory supported by experiments and
mathematical deductions.
The Atomic Philosophy of Early Greeks
Leucippus of Miletus is thought to have originated the atomic philosophy. His famous disciple,
Democritus of Abdera, named the building blocks of matter. He believed that atoms were
uniform, solid, hard, incompressible, and indestructible and keep moving in empty space in
infinite numbers till stopped. Differences in atomic shape and size determined the various
properties of matter. In Democritus’s philosophy, atoms existed not only for matter but also
for such qualities as perception and the human soul. For example, sourness was supposed to
be caused by needle-shaped atoms while the color white was believed to be composed of smoothsurfaced atoms. Very fine atoms were considered to be forming the soul.This atomic philosophy
was developed as a middle ground between two opposing Greek theories about reality and
illusion of change. He argued that matter was subdivided into indivisible and immutable
particles that created the appearance of change when they joined and separated from others.
The philosopher Epicurus of Samos (341–270 B.C.) used Democritus’s ideas subside
the fears of superstitious Greeks.
According to Epicurus’s materialistic theory, the entire universe was composed of atoms
and voids and so even the gods were subjected to Laws of Nature.
The great Latin poet Titus Lucretius Carus (57 B.C.) gave a faithful picture of the
atom as solid seed in his long poem ‘De Rerum Natura’(‘On the Nature of Things’),
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as quoted partly here:
“But solid seeds exist. Which fill their place;
And make a difference between full and space.
These, as I proved before, no active flame,
No subtle cold can pierce and break their frame.
Tho’ every compound yields: no powerful blow,
No subtle wedge devide, or break in four.
For nothing can be expose,
No part destroyed by force;
Or cleft without a void,
And things that hold most void,
When strokes do squeeze,
Or subtle wedges enter, yield with ease.
If seeds then solid are, they must endure
Eternally, From force, from stroke secure.”
That was the explanation of an atom two thousand years ago. The Greek atomic theory
is significant historically and philosophically, it has no scientific value as it was not based on
observations of nature, measurements, tests, or experiments. Instead Greeks used mathematics
and reason almost exclusively when they wrote about science. They wanted an all-encompassing
theory to explain the universe and not merely a detailed view of as tiny portion of it as an
atom. Thus Plato and Aristotle attacked Democritus’s theory of atom on philosophical grounds
rather than on scientific grounds. Plato valued abstract ideas more and so he rejected the
notion that attributes qualities like beauty and goodness to “mechanical manifestations of
material atoms”. Democritus believed that matter can only move through a vacuum and light
was the rapid movement of particles through a void. Aristotle rejected the existence of vacuum.
The Emergence of Experimental Science
The poem De Rerum Natura, which was rediscovered in the 15th century helped fuel a debate
between orthodox people belonging to Roman Catholics, guided by Aristotle’s philosophy and
new scientific experimenters in 17th century. The poem was popularized by Pierre Gassendi
a French priest who tried to separate Epicurus’s atomism from its materialistic background
by telling that God created the atoms.
Today, as science has advanced so much, chemists and physicists have turned their
thoughts towards the atom.
Soon after Galileo Galilei expressed that vacuums can exist, scientists began studying
properties of air and partial vacuums to test the relative merits of orthodox views and the
atomic theory.
Robert Boyle (1627–91)an Anglo-Irish chemist and Sir Isaac Newton (1642–1726)
are well known scientists to the world, who were the first to suggest that matter was composed
of tiny particles which could not be split up into smaller particles arranged into molecules to
give material its different properties were at man’s disposal. In the early 18th century, Sir
Isaac Newton expressed his views of the atom that was similar to that of Democritus Gassendi
and Boyle.
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The English chemist and physicist John Dalton converted the atomic philosophy of the Greeks
into a scientific theory between 1803 and 1808. His book A New System of Chemical
Philosophy (part I in 1808 and part II in 1810) was the first application of atomic theory to
chemists.
1.1
DALTON’S ATOMIC THEORY
As a result of his experiments on gases, John Dalton (1766–1844) gave to the famous theory
(1803), which in simple language is as follows:
(i) All elements are made up of minute particles (he named them atoms) which are
indestructible and impenetrable.
(ii) The atoms are identical in weight, size and all other physical and chemical properties
for one given element, but differ from those of the other elements.
(iii) Chemical combination occurs through the union of these atoms in simple numerical
ratio.
The above theory provided a physical picture of how elements combine to form compounds
and a phenomenological reason for believing that the atoms exist.
Since no other scientist of the above period presented any conflicting evidence, the
theory was accepted until the close of 19th century. It is not surprising that the theory provided
the foundation for the remarkable progress made in this new branch of science, dealing with
Nuclear Energy.
Size of the Atom
The first estimates of the size of atoms and the number of atoms in a given volume were made
by the German chemist Joseph Loschmidt in 1865. He used the results of Kinetic theory
and some rough estimates to do his calculations. The size of the atoms and the distance between
them in the gaseous state are related both to the contraction of the gas upon liquefaction and
to the mean free path traveled by the molecules in the gas. Loschmidt found the size of the
atom and spacing between the atoms by finding a common solution to these relationships. His
result for the number of atoms in 12 grams of the carbon was 6.022 × 1023 which was remarkably
close to the presently accepted value of Avogadro’s number. His result for the diameter of
an atom was approximately 10–8 cm.
1.2
INTRODUCING THE ATOM
How was any one going to be able to pry into anything so small as an atom? We are told that
if these tiny particles are placed side by side, 250,000,000 Hydrogen atoms would make a
queue one inch long. (The diameter of a Hydrogen atom is found to be 10–9 cm, and that of the
nucleus is 10–14 cm.)
Up to the last century scientists believed that the atom was impenetrable and indivisible.
But the accumulating experimental evidence suggested that this view was incorrect.
CHAPTER 1
Modern Atomic Theory
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Discovery of Electron
In 1858, Julius Plucker, on passing an electric discharge through a gas at low pressure in a
vacuum tube (Fig. 1.1), observed phosphorescence near the anode. It was a strange looking
glass tube with two electrodes. These electrodes were metal plates called the cathode and
anode, which were connected to a source of high voltage. The tube also had an arrangement
for evacuating air, in order to obtain a high vacuum or very low pressure inside the tube. This
visible discharge was named ‘cathode rays’ as it originated from cathode. The English
physicist and chemist William Crookes investigated cathode rays in 1879 and found that
they were bent by a magnetic and electrical fields. J. J. Thomson found in 1897 that the
deflection was proportional to the difference in potential between the aluminium plates serving
as electrodes. His discovery also established the particulate nature of cathode rays. The direction
of the deflection suggested that they were negatively charged particles. Accordingly, he named
the particles electrons. The cathode rays consisted of a stream of small particles like miniature
bullets shot from the cathode at terrific velocity, in a straight line and have high penetrating
power. Each of these particles was observed to have a negative charge which was equal but
opposite to that of the hydrogen ion, and a mass which was approximately 1/1840 of that of the
hydrogen atom; i.e., put a decimal point before 27 zeros and then the numbers 9. This is the
mass in grams. Each particle has a diameter 1/40,000 of an atom. These particles are now
called electrons, and they are the universal constituent of all matter.
Cathode
rays
Gas at
low pressure
To vacuum
pump
Cathode
–
+ Anode
High voltage
+
Fig. 1.1. Electric discharge tube
From the magnitude of the electrical and magnetic deflections, Thomson could calculate
the ratio of charge to mass (e/m) for the electrons. This ratio was known for the atoms from
electrochemical studies. This e/m ratio was the same for any gas taken in the Crooke’s tube.
Measuring this and comparing it with that for an atom Thomson discovered that the mass of
an electron is very small, merely 1/1836 that of a hydrogen ion. When scientists realized
that an electron is virtually 2000 times lighter than the smallest atom—hydrogen, they
understood how cathode rays could penetrate metal sheets and how electric current flows
through copper wires. In deriving the e/m ratio, Thomson had calculated the velocity of electron.
It was 1/10 the speed of light, thus amounting to roughly 30,000 km per second.
Thus, the first subatomic particle was identified, the smallest and the fastest bit of
matter known at that time.
These electrons do a lot of things for us. They constitute the electric current, they hop
inside a radio valve, they sweep across a cathode tube to show us pictures in television, etc.
About six billion electrons pass through the filament of an ordinary 100 watt lamp each second.
In 1895, Wilhelm K. Rontgen observed that when a hard substance like tungsten (W)
was bombarded by cathode rays (Fig. 1.2), a very penetrating radiation was obtained. He
named this radiation ‘X-rays’. These rays are similar to light rays, but they have much smaller
wavelength of the order of 1/5000 of the wavelength of visible light. These rays easily passed
through a variety of substances, including human flesh. When Rontgen placed his hand in the
path of the rays, he saw the clear image of the bones on the fluorescent screen.
Cathode rays
+
–
Cathode
Anode
Metal target
Diffracted
X-rays
X-ray beam
Diffracting unit
Fig. 1.2. An X-ray tube
Robert Andrews Milliken an American scientist greatly improved the method
employed by Thomson for measuring directly the charge of an electron. In Milliken’s oildrop experiment (1909-1910), he produced microscopic droplets of an oil and observed them
falling in the space between two electrically charged plates. Some of the droplets became
charged and could be suspended by a delicate adjustment of the electric field. Milliken knew
the weight of the droplets from their rate of falling when the electric field was turned off.
From the balance of the gravitational and electrical forces, he could determine the charge on
each the droplets. All the measured charges were integral multiples of a quantity
1.60217733 × 10–19 coulomb. This experiment was the first to detect and measure the effect of
an individual subatomic particle. For this work Milliken was awarded the Nobel Prize in
1923.
Discovery of Proton
In 1886, Goldstein used energized discharge tube also called Crooke’s tube having perforated
cathode (Fig. 1.3) containing a gas at a very low pressure and on passing high voltage found
that, some rays coming from the side of anode passed through the holes in the perforated
cathode and produced a green fluorescence on the opposite glass wall coated with Zinc Sulphide.
These rays were called ‘anode rays’ or ‘canal rays’. These rays were produced as a result of
knocking off the electron from gaseous atoms by the impact of high speed electrons of cathode
rays on them. The anode rays were made up of the positively charged particles. The German
scientist Wilhelm Wien analyzed these rays in 1898 and found that the particles have different
e/m ratio for different gases taken in the discharge tube. Moreover, e/m ratio is about 1/1000
times that of an electron. Because the ratio of the particles is also comparable to the e/m ratio
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of the residual atoms in the discharge tube, the scientists thought that the rays were actually
ions from the gases inside the tube. The mass of anode ray particle was found to be different
for different experimental gases. Further experiments revealed that when hydrogen gas
was taken inside the discharge tube, the particles present in anode rays have a minimum
mass. These particles were termed protons. The mass of proton is nearly the same as that of
an atom of hydrogen, i.e., 1.67 × 10–24 grams. The charge on the proton is equal in magnitude
to that on the electron, i.e., 1.602 × 10–19 coulomb but opposite in nature. The attractive force
between positively charged protons and negatively charged electrons keeps electrons moving
in orbits around the nucleus, something like the way the gravity keeps the Earth in orbit
around the sun.
To vacuum pump
ZnS
coating
+
–
Perforated
cathode
H2 gas inside
at low pressure
Fig. 1.3. Anode rays or canal rays
Charge, Mass, Spin and Size
The electron has a mass of about 9.1093897 × 10–28 grams. The mass of a proton or a neutron
is about 1,836 times more than the electron. The mass of proton is 1.67 × 10–24 grams and the
charge of the proton and electron is ± 1.602 × 10–19 coulomb. Neutron has no charge and it is
slightly more massive than a proton. The atom as a whole is electrically neutral as it contains
equal number of protons and electrons.
All atoms are roughly the same size, whether they have 3 or 90 electrons. Approximately,
50 million atoms of solid matter lined up in a row would measure only 1 cm. A convenient unit
of length for measuring atomic sizes is the angstrom (Å) which is equal to 10–10 meter. The
radius of an atom measures 1 to 2 angstroms. Compared with the overall size of the atom the
nucleus is much tinier. It is in the same proportion to the atom as the marble is to a football
ground. The volume of a nucleus is about 10–14 meters. Thus, it occupies 1/100,000 part of an
atom. Hence the more practical unit of length for measuring nuclear sizes is the femtometre
(fm) which is equal to 10–15 meter. The diameter of the nucleus depends on the number of
particles it contains and it ranges from 4 fm for light nucleus like carbon to 15 fm for the
heavy nucleus like lead. In spite of the small size of the nucleus, virtually all the mass of the
atom is concentrated there. The lightest nucleus, that of hydrogen, is only 1836 times heavier
than electron, but some heavy nuclei are nearly 500,000 times more massive than the electron.
Life of Particles
Protons, neutrons and electrons are long-lived particles. Other subatomic particles are
also known now, but they are short-lived. We shall discuss about them in a separate chapter
in this book.
The electron has other intrinsic property like spin. The electron cannot spin in any
arbitrarily manner, but spins only at a certain specific rates governed by Quantum Mechanics.
These rates can be 1/2, 1, 3/2, 2, 5/2,—times a basic unit of rotation. Like protons and neutrons,
electrons have a spin equal to 1/2.
Atomic Number
The single most important characteristic of an atom is its atomic number usually denoted
by Z. Atomic number is defined as the number of units of positive charge in the nucleus. It is
equal to number of protons in the nucleus. For example, if an atom has Z equal to 11, it is
sodium, while Z equal to 92 corresponds to uranium.
Model of an Atom
Some five centuries before christ that is from Democritus’s time, many theories for structure
of atom have been suggested, argued upon and rejected. Much later on there was rather
compelling evidence that matter is composed of relatively few building blocks that we refer to
as atoms and matter could be described by an atomic theory. Since then, a few of the models
gain popularity in the course of time.Although atomic models has taken years and years to
build, some of them are as follows:
•
Indivisible atom model: After the Dalton’s theory was put up in 1803, the atom
was percieved as a hard sphere which is indivisible, impenertable and indestructible.
Indivisible atom
(Hard sphere)
•
Plum-pudding model: In 1897, J.J.Thomson
showed that electrons have negative electric charge
–
–
and come from ordinary matter. For matter to be
electrically neutral, there must also be an equal
Distributed
positive charge
amount of positive charge lurking somewhere.
–
A prevailing model of the time of J.J.Thomson, it
–
–
was proposed that negatively charged particles
(electrons) were scattered like plums in the
smeared- out positive charges (the pudding).The
Plum-pudding model of an atom
model explained the neutrality of the bulk material,
yet allowed the description of flow of elecrtic current. In this model, it would be very
unlikely for an alpha particle to scatter through an angle greater than a small fraction
of a degree, and a vast majority should undergo almost no scattering at all. (Refer to
Rutherford’s experiment of scattering of alpha particles described on page 13).
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•
Cubical atom model: Some scientists suggested a cubical shape for an atom. But
this did not gain any support due to lack of evidence in favour of it.
•
Rutherford’s model: The results from Rutherford’s
experiment(see page 13) were astounding. The vast majority
of alpha particles behaved as expected, and hardly scattered
e
at all. But there were alpha particles that scattered through
angles greater than 90 degrees, incredible in the light of
expectations for a plum-pudding like model of an atom.It
was largely the evidence from this type of experiment that
led to a model of the atom as having a very tiny, compact
Rutherford’s model
central core(nucleus) that houses the entire positive charge
and most of the mass of the atom, while the majority of the
atom’s volume contains descrete electrons orbiting around
the central nucleus. In Rutherford’s model the electrons moving in the orbits around
the nucleus can be likened to our solar system in which the planets move in orbits
around the sun.
Bohr model: Under classical electromagnetic theory, a charged body that is moving
in a circular path, loses energy. However, in Rutherford’s model, there was nothing
to prevent the electrons from loosing energy and finally falling into the nucleus under
the electrostatic force of attraction. (See page 31 also).This stability problem was
solved by Neil Bohr in1913, with a new model in which there are particular orbits
in which electrons move without loosing energy and therefore do not spiral into the
nucleus. This model was based on quantum mechanics, which successfully
explained most of the behaviour of atoms. Bohr’s model is still the convenient
description of the energy levels of the atom. (See the details on page 31).
•
•
•
Wave mechanical model: This model is widely accepted today and this has left all
the above models behind. This model is described in detail on pages 36 to 53.
String theory model: It is a very recent model, based on a similar idea of a stretched
string and vibration governing a subatomic particle’s properties.This is being
developed at Atomic Research centre at the Department of Physics UWA. A little
about this theory is in the chapter of subatomic particles.