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Transcript
ATOMS: Dalton and Beyond
A search for a simple theory of
matter
Topic 7 – Spring 2005
Ted Georgian, Dept. of Biology
1
The nature of science
Scientists are searching for explanations that are:
1.
2.
3.
2
Models in science
Scientists use their experimental results
(and their imaginations)
to create models
A model is a representation of a complex natural
system that permits us to understand its behavior.
These models may be mental,
mathematical, or even physical.
3
Prior Examples?
Remember the models of planetary motion?
What were they trying to explain?
(These animated gifs were obtained from Dr. Stephen J. Daunt's Astronomy 161
web site at The University of Tennesee, Knoxville.)
4
Early Greek atomists
Leucippus
(~480 - 420
B.C.)
http://cont1.edunet4u.net/cobac2/down/dow
n05.html
• There is only one type of
matter, found in tiny,
indivisible particles called
“atoms”
• All change is caused by
atoms moving through
empty space
Democritus (470 - 380 B.C.)
www.livius.org/a/ 1/greeks/democritus.jpg
• Atoms are therefore
“fundamental”
5
But an alternate model won out
http://astsun.astro.virginia.edu/~jh8h/Foundations/ch
apter2.html
Aristotle (384 – 322 BC)
6
The mechanical philosophy of the 1600s
• Descartes, Boyle
and Newton
• A “clockwork”
universe
• Simplicity,
generality, and
precise predictions
7
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
8
Start of the Modern Era of Atoms
John Dalton’s Atomic
Hypothesis (1803):
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
9
Dalton’s Atomic Model of Compounds
• explained observation of
“constant proportions” as
based on atomic composition
of compounds
• used “Rule of greatest
simplicity” to guess at the
atomic structure of
compounds
• estimated relative atomic
masses, based on his
hypothesized structures
10
Meanwhile, many new elements being found
11
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.
12
Dmitri
Mendeleev
(1834-1907)
“Creator of the
Periodic Table”
(but there were earlier attempts by
Dobereiner and Newlands, and
Meyer probably formulated the
periodic idea at same time as
Mendeleev)
13
Mendeleev’s
early notes
for the
Periodic Table
(1869)
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
Characteristics of Mendeleev’s Table
• Organized 60+ known elements…
- by similar properties in each vertical family
(group)
- by roughly increasing atomic weight within
each horizontal row (moved 17 elements
based on properties rather than weight)
• Used to predict existence of new elements
(of 10, found 7; other 3 do not exist)
16
Prediction of the properties of an unknown
Group 4 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
17
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)
18
News flash: atoms aren’t fundamental
J. J. Thomson (1897)
• experimented with “cathode rays”
• “and then... made a bold speculative
leap. Cathode rays are not only
material particles, he suggested, but in
fact the building blocks of the atom:
they are the long-sought basic unit of
all matter in the universe.”
(http://www.aip.org/history/electron/jjrays.htm)
Schematic of actual
1897 apparatus
(vacuum inside):
19
Cathode-Ray Tubes – ever seen one?
http://www.howstuffworks.com/tv4.htm
20
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."
• “I can see no escape from the conclusion that [cathode rays]
are charges of electricity carried by particles of matter.”
but...
• “What are these particles? Are they atoms, or molecules, or
matter in a still finer state of subdivision? - J. J. Thomson
21
Thomson’s “plum pudding” atom model*
Cathode rays
(electrons) are...
• tiny “corpuscles”
of negative charge
• surrounded by a
sort of “cloud” of
positive charge
* Never had plum pudding? Think of a blueberry muffin.
22
If electrons exist, how big are they?
• Thomson calculated the mass-to-charge ratio for
cathode ray particles: it was over 1000 times smaller
than for a charged hydrogen atom
• This fact suggested:
- either cathode rays carried a huge charge,
- or they had very small mass
• Robert Millikan measured the charge of a cathode ray
particle in 1910. From that he could calculate the
mass: ~1800 times lighter than a hydrogen atom
23
More pieces of the atom
Ernest Rutherford
(1871-1937)
nuclear physicist,
Thomson’s student,
New Zealander teaching
in Great Britain
Gold Leaf Experiment
24
Rutherford’s Experiments (1910-11)
(done by undergrad Ernest Marsden/physicist Hans Geiger)
• Fired beam of positively-charged alpha particles at
very thin gold foil.
• Alpha particles caused flashes of light when they hit
the zinc sulfide screen
25
Rutherford’s 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
Rutherford’s Experiment:
prediction
Alpha particles will pass
through like this …
27
Rutherford’s experiment: what
actually happened
28
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
Expt. Interpretation:
• gold atom has small,
dense, positively-charged
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 a marble, the
innermost electrons would be
how far away?
•
•
•
•
+
One-half inch
Six inches
Eighteen inches
One-half mile
(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”
Atom “split” by John Cockcroft and
Ernest Walton, using a particle
accelerator, in late 1932
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
Properties of Subatomic Particles
Property
Particle
Electron
Proton
Neutron
Mass (amu),
Mass (g)
0.00055
9.110 x 10-28
1.00728
1.673 x 10-24
1.00866
1.675 x 10-24
Relative
Charge
-1
+1
0
36
Now we understand why the
elements come in periods of 8
• 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. A given
element can have different number of neutrons,
and therefore different atomic masses.
• The chemical properties of the elements are
determined by the number of electrons in their
outer (valence) shells
37
Why do 2 Group I atoms combine
with 1 oxygen (R2O)?
38
Modern Periodic Table Organization
• Elements are NOW placed in order of
increasing atomic number (# of + protons).
- Why? Gives absolute order...
atomic weights not characteristic
(different-mass atoms called isotopes exist!)
• A relationship between nuclear charge and
arrangement of elements in the Table was
finally discovered in 1914 (Henry Moseley).
• In 1860s, Mendeleev could NOT have predicted
a relationship to subatomic particles!
39
So: is this what atoms are like?
No!
Electrons moving through
the electrical field generated
by the protons in the nucleus
would radiate away energy
and spiral down into the
nucleus
Calculations soon showed that a
“Rutherford atom” would last
less that one minute.
40
A new understanding of the atom
It had long been know that
when chemical elements
are heated, they gave off
light of a particular
wavelength (or color)
Sodium Potassium Lithium
41
Spectroscopes: Seeing Atomic Light
Original 1859
BunsenKirchhoff
spectroscope
Typical setup
for viewing a
line-emission
spectrum
42
Spectroscopy can identify elements on
distant stars
Hydrogen
Helium
http://heasarc.gsfc.nasa.gov/docs/xmm_lc/edu/lessons/student-worksheet-spectragraph2.html
Helium was discovered in a spectrograph of the
Sun in 1868 and not on Earth until 27 years later.
43
Hydrogen’s Emission “Fingerprint”
Observation:
when hit with electricity
hydrogen gives off light
of specific wavelengths,
NOT continuous range!
The line-emission spectrum
of hydrogen gas
(the bands visible to humans)
44
Niels Bohr
(1885-1962)
Danish physicist
Bohr wondered why
hydrogen emitted spectral
lines, and not just a
continuous band of light
45
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
• Circling electron maintains orbit ONLY at specific
distances from nucleus
• Only way electron could exist for long time without
giving off radiation
• Bohr’s model enabled him to predict the number and
wavelength of hydrogen’s emission lines
46
Electron orbits are distinct
(“quantized”) in Bohr’s model
Trefil & Hazen. The Sciences: An integrated approach. 2 nd ed. Fig. 7-6.
47
But why should electrons behave this way?
Louis de Broglie (1927)
Particle/Wave Duality of electrons
Thus I arrived at the following general idea which has guided
my researches: 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. In
other words, in both cases we must assume the existence of
corpuscles accompanied by waves.
De Broglies Nobel Prize speech, 1927. http://www.spaceandmotion.com/Physics-Louis-de-Broglie.htm
48
Electrons can be thought of as
standing waves …
49
Electrons as waves
Only at certain distances from the
nucleus would the electron
complete an integer number of
wavelengths in its movement
around the nucleus
When the mathematics was worked out, these
distances agreed exactly with those assumed by
Bohr for the hydrogen atom.
50
The position of electrons can’t be
predicted precisely
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
51
Electrons move in “probability
clouds”, not circular orbits
• The exact path of an
electron can’t be
predicted
• If we know the
electron is somewhere
in the atom, it’s
velocity is uncertain
by ~7,300 km/s (~ 16
million mph)!
52
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.”
53
Here we go again!
By the 1950s hundreds of sub-atomic particles had
been identified. Simplicity was getting lost again.
54
Another attempt to simplify our model
of matter
Murray Gell-Mann and George Zweig (1964) proposed protons and neutrons are made of smaller
particles they named quarks (aces)
55
Protons & neutrons are not fundamental
• Gell-Mann & Zweig hypothesized 6 different quarks
with fractional charge (UP quark has +2/3 charge,
DOWN quark has –1/3)
• Protons and neutrons are composed of UP and
DOWN quarks, held together by gluon particles
56
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)
57
So: are quarks fundamental?
Probably not: recent models of matter hypothesize
11-dimensional “strings” curled up inside of
quarks.
58