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ATOMS: Dalton and Beyond
A search for a simple theory of
matter
Topic 7 – Spring 2006
Ted Georgian, Dept. of Biology
1
The nature of science
Scientists are searching for explanations that are:
1.
2.
3.
2
What is the world made of, at the
most fundamental level?
www.ikros.net/mocs/ images/Castle2Clouds.jpg
3
Early Greek atomists
Leucippus
(~480 - 420
B.C.)
http://cont1.edunet4u.net/cobac2/down/dow
n05.html
• All matter is made of tiny,
indivisible particles called
“atoms”
• Change is caused by atoms
moving through empty
space (a “void”)
• Atoms are therefore
“fundamental”
Democritus (470 - 380 B.C.)
www.livius.org/a/ 1/greeks/democritus.jpg
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But an alternate model won out
http://astsun.astro.virginia.edu/~jh8h/Foundations/ch
apter2.html
Aristotle (384 – 322 BC)
Based on observable
characteristics of matter,
such as
?
5
The mechanical philosophy of the 1600s
Descartes, Boyle, Newton & others imagined a “clockwork”
universe - perfectly predictable
6
Would it work for chemistry as well?
Maybe chemistry would turn out to be as
“simple” as Newtonian physics?
A few, simple objects following
simple, general, and precise
laws.
7
Start of the Modern Era of
Chemistry
John Dalton’s Atomic
Hypothesis (1808):
1. All matter is made up of
indivisible atoms.
2. Compounds are composed of
atoms in definite proportions.
3. Chemical change occurs
when atoms are rearranged
8
Dalton’s Atomic Model of Compounds
• explained observation of
“constant proportions”
• used hypothesis (“Rule”) of
greatest simplicity
• estimated relative atomic
masses
9
Meanwhile, many new elements being found
10
How to make sense of all these
elements?
Scientists like “a place for
everything, and
everything in its place.”
And no more places and
things than necessary.
11
Dmitri
Mendeleev
(1834-1907)
“Creator of the
Periodic Table”
(but there were earlier
attempts by others)
12
Mendeleev’s
early notes
for the
Periodic Table
(1869)
13
Characteristics of Mendeleev’s Table
• Organized 60+ known elements…
- by similar chemical properties in each
vertical family (group)
- by roughly increasing atomic weight within
each horizontal row
• Used to predict existence of new elements
(of 10, found 7; other 3 do not exist)
14
Mendeleev’s table,
as originally
published
• Formatted
sideways
compared to
modern table
• ? instead of a
name: element
was predicted to
exist but not
known yet
15
Prediction of the properties of an unknown
element below Silicon
*
Property
Observed
for Si
Predicted Observed
for eka-Si
for Sn
Atomic
mass
28
72
118
72.6
Density
(g/cm2)
2.33
5.5
7.28
5.35
Formula
of oxide
SiO2
Eka-SiO2
SnO2
GeO2
Formula
of
chloride
SiCl4
Eka-SiCl4
SnCl4
GeCl4
eka: “one beyond”
Observed
for Ge
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An attempt to simplify the elements
William Prout (1815)
• hypothesized that the hydrogen
atom is fundamental
• all other elements made up of
hydrogen atoms
• his hypothesis was rejected by the
1830s (for ex. chlorine atom had
mass 35.4 times that of hydrogen)
17
News flash: a new type of matter is
discovered
J. J. Thomson (1897)
• experimented with “cathode
rays”
• decided that they are charges of
electricity carried by particles of
matter
Schematic of actual
1897 apparatus
(vacuum inside):
18
Cathode-Ray Tubes – ever seen one?
http://www.howstuffworks.com/tv4.htm
19
Thomson’s conclusions
• “We have, in the cathode rays,
matter in a new state...a state
in which all matter...is of one
and the same kind; this matter
being the substance from which
all the chemical elements are
built up."
but...
• “What are these particles? Are they atoms, or
molecules, or matter in a still finer state of
subdivision?” - J. J. Thomson
http://www.aip.org/history.electron/jjrays.htm
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How big are “electrons”?
• Thomson calculated the mass-to-charge ratio for
cathode ray particles: it was over 1000 times smaller
than of a charged hydrogen atom
• This fact suggested:
- either cathode rays carried a huge charge,
- or they had very small mass
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Answer: very, very small
• Robert Millikan measured the
charge of a cathode ray particle
in 1910.
• From that & Thompson’s massto-charge ratio, he could
calculate the mass: ~1800
times lighter than a hydrogen
atom
22
Thomson’s “plum pudding” atom model*
• tiny “corpuscles”
of negative charge
• surrounded by a
sort of “cloud” of
positive charge
* Never had plum pudding? Think of a blueberry muffin.
23
More pieces of the atom
Ernest Rutherford
• Thomson’s student
• Gold Leaf Experiment
(1910-11) – actually
conducted by Hans Geiger
and undergraduate Ernest
Marsden
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The gold leaf experiment
• fired positively-charged alpha particles at very thin gold foil
– they caused flashes of light when they hit the screen
• counted flashes and measured the angle of deflection
25
Gold leaf experiment: prediction
By Thomson’s model,
mass and + charge of gold
atom are too dispersed to
deflect the positively-charged
alpha particles,
so...
particles should shoot straight
through the gold atoms.
26
like this:
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What actually happened:
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What’s going on?
Most alpha particles went
straight through, and
some were deflected,
BUT
a few (1 in 20,000) reflected
straight back to the source!
“It was quite the most incredible event that has ever happened
to me. It was almost as incredible as if you had fired a fifteen inch
shell at a piece of tissue paper and it came back and hit you.”
29
Rutherford’s Model of the Atom
• small, dense, positivelycharged nucleus
surrounded by “mostly
empty” space
in which the electrons must
exist.
+
• positively charged particles
called “protons”
• like tiny solar system
30
The nucleus repels alpha particles
+
31
How much of an atom is empty space?
Most of it!
In fact, if the nucleus of an atom
were the size of Murphy
Auditorium, the innermost
electrons would be how far
away?
A.
B.
C.
D.
+
DeLaRoche Hall?
Francis Hall?
Downtown Olean?
NYC?
(click for the right answer)
32
But wait – there’s more!
James Chadwick
(1932)
•
•
discovered a neutral
(uncharged) particle in the
nucleus.
called it the “neutron”
33
Atom “split” later that year
Atom “split” by
John Cockcroft and
Ernest Walton, using
a particle
accelerator, in late
1932
34
Splitting the atom led to some very
practical consequences
35
Now we understand why the
periodic table works
• The order of the elements is determined by their
atomic number (= the number of protons)
• The atomic mass of the elements is determined
by the number of protons and neutrons.
• The chemical properties of the elements are
determined by the number of electrons in their
outer (valence) shells
36
Why do 2 Group I atoms combine
with 1 oxygen (R2O)?
37
38
So: is this what atoms are like?
No!
Calculations soon showed
that a “Rutherford atom” would
last less than one minute
Electrons would radiate away
energy and spiral down into the
nucleus.
39
A new understanding of the atom
from spectroscopy
When elements are heated,
they give off light of a
particular wavelength
(or color)
Sodium Potassium Lithium
40
Spectroscopes: seeing atomic light
Original 1859
BunsenKirchhoff
spectroscope
Modern
apparatus
for viewing a
“spectrum”
41
Hydrogen’s emission “fingerprint”
Observation:
when heated with electricity
hydrogen gives off light
of specific wavelengths
The line-emission spectrum of hydrogen gas
42
Niels Bohr
(1885-1962)
Danish physicist
Bohr wondered why
hydrogen emitted spectral
lines, and not just a
continuous band of light
43
Bohr’s Model of Atom (1913)
H's electron
r1
r2
The first three allowed energy levels,
at distances r1, r2, and r3 from nucleus.
r3
H's nucleus containing 1 proton
• Bohr assumed that electrons can orbit ONLY at
certain distances from nucleus
• this model permits electrons to exist for a long time
without giving off radiation
• Bohr’s model enabled him to predict the number and
wavelength of hydrogen’s emission lines
44
Electron orbits are distinct
(“quantized”) in Bohr’s model
“Quantum
leaps”
from one level to
another
Trefil & Hazen. The Sciences: An integrated approach. 2 nd ed. Fig. 7-6.
45
But why should electrons behave this way?
Louis de Broglie (1927)
Particle/Wave Duality
of electrons
“Thus I arrived at the following general idea …: for matter,
just as much as for radiation, in particular light, we must
introduce at one and the same time the corpuscle
concept and the wave concept.”
http://www.spaceandmotion.com/Physics-Louis-de-Broglie.htm
46
Electrons as waves
Only at certain distances from
the nucleus would an
electron complete an
integer number of
wavelengths in its orbit
When de Broglie did the mathematics, he could
predict exactly the distances that Bohr had
assumed for the hydrogen atom.
47
Then, suddenly, trouble for the
mechanistic approach
Werner Heisenberg (1927)
The “Uncertainty Principle”
• There’s an upper limit to how precisely an electron’s
position and momentum can be known
• The more precisely one is known, the less precisely
the other can be known
48
Electrons move in “probability
clouds”, not circular orbits
• The exact path of an
electron can’t be
predicted!
• If we know the electron
is in a given atom, its
velocity is uncertain by
~16 million mph!!
49
Newtonian certainty cannot be
obtained in the subatomic world
“I cannot believe that God
plays dice with the
universe.”
“Albert, stop telling God
what to do.”
50
Here we go again!
By the 1950s hundreds of sub-atomic particles had
been identified. Simplicity was getting lost again.
51
Another attempt to simplify our model
of matter
Murray Gell-Mann and George Zweig (1964) proposed that protons and neutrons are made of
smaller particles they named quarks (aces)
52
Gell-Mann & Zweig hypothesized 6
different quarks
• Quarks have fractional
charges
• Then how do particles in
the nucleus end up with
+1 or zero charges?
53
Protons & neutrons are not fundamental
Protons and neutrons are composed of UP and
DOWN quarks, held together by gluon particles
54
Fermi National Accelerator Lab:
6-km Tevatron ring and 3-km Main Injector *
• Chicago site for
study of subsubatomic
particles
• Evidence for last
quark (TOP)
found in 1995
*contrast to world’s-largest machine: CERN 27-km
LEP collider (1989-2000)
55
So: are quarks fundamental?
Probably not: recent models of matter hypothesize
11-dimensional “strings” curled up inside of
quarks …stay tuned for future developments.
56