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2/5/2014
Early Atomic Theory
A Historical Perspective
What do Dinosaurs and Atoms have in Common?
No one has seen an atom or a dinosaur directly.
We know of their existence only by indirect
evidence. Our theories of both dinosaurs and
atoms has changed over time based on this
indirect evidence
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Democritus, Greek philosopher, 460 to ca. 370 BC held
that everything is composed of "atoms", which are
invisible, indestructible fundamental units of matter
• that between atoms lies empty
space;
• that atoms are indestructible; have
always been, and always will be, in
motion;
• that there are an infinite number of
atoms, and kinds of atoms, which
differ in shape, and size.
• Matter is composed of tiny particles
called atoms
Titus Lucretius Carus (died c. 50 BC) was an
Epicurean poet writing in the middle years of
the first century BC. He wrote a six-book Latin
hexameter poem De rerum natura translated
On the nature of things
The universe is infinite,
Lucretius argues,
consisting of infinitely
extended space and an
infinite number of atoms.
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What is an ‘Atom’
• The smallest particle in
an element that retains
its identity in a
chemical reaction.
• From the Greek
adjective atomos or
atomon, ‘indivisible’
PIECES
OF
CARBON
CARBON
ATOM
Antoine Lavoisier (France 1782)
• Law of Conservation
of Mass
• In a chemical reaction
mass is conserved.
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His experiments…
When mercury is heated in oxygen at a moderate
temperature, a red substance, calx of mercury,
(mercuric oxide) is obtained. (A calx is the ash left when
a substance burns in air.) At a higher temperature this
calx decomposes into mercury and oxygen.
2 Hg(l) + O2(g) → 2 HgO (s)
His experiments also revealed that
the combined masses of mercury
(200.59) and oxygen (16.00) were
exactly equal to the mass of calx of
mercury (216.59). That is, there was
no change in mass upon formation
or decomposition of the calx.
Lavoisier hypothesized that this
should be true of all chemical
changes, and further experiments
showed that he was right. This
principle is now called the law of
conservation of mass
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Joseph Proust (France 1754-1826) Based on his
studies on copper carbonate reactions, Proust
discovered that each pure compound has its
own characteristic elemental composition
Law of Definite Proportions:
The elements that comprise a
compound are always in a fixed
proportion by mass.
H2O
2 g/mol : 16 g/mol
John Dalton (England 1766-1844)
• School teacher
• Studied the ratios in which
elements combine in
chemical reactions
• Formulated the first
modern Atomic Theory
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Dalton’s Atomic Theory
1. All matter is made of atoms.
2. Atoms of the same element are identical.
The atoms of any one element are
different from those of any other
element.
3. Atoms of different elements can
chemically combine in simple whole
number ratios to form compounds.
Example: CO2
Dalton’s Atomic Theory
4. Chemical reactions occur when atoms are
rearranged.
Chemical reactions do not change atoms of
one element to another.
Dalton’s Atomic model
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Atomic Theory
• Much of Dalton’s theory still holds today
• However, we now know that atoms are not indivisible
Fermi Lab
http://www.youtube.com/v/IrrxNxDKtd8
Particle collider
Result of particle
collisions
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Scanning Tunneling Microscope (STM)
http://www.youtube.com/v/BUKTIwL1Uy8
Scanning Tunneling Microscope (STM)
The scanning tunneling
microscope (STM) is an
instrument with which
one can image surfaces
so finely that individual
atoms are resolved. The
STM can even move
atoms about. The
images were obtained
by scanning the surface
with the same
instrument.
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Iron Atom Arrangement - STM
Atomic Force Microscope (AFM)
is one type of scanning probe
microscopes, which is used to
image surface structures (on a nm
or even sub-nm scale scale) and
to measure surface forces
http://www.youtube.com/v/MZb8C0f7Kdg
http://www.youtube.com/v/Ha53tFTsmW8
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What is the size of an atom?
• If you placed
100,000,000 Cu
atoms side by side
they would form a
line only 1 cm long
• Radius of most
atoms is about
5x10-11 to 2x10-10m
STM Image
The Discovery of the Electron and the Proton
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History
• Electron means “amber” in Greek
• Properties discovered by the Greek Thales of
Miletos 600 BC. Rubbed the mineral amber
with cat fur and attracted feathers.
Benjamin Franklin (America 1740’s)
• Law of conservation of
charge.
• Viewed electricity as a
flowing fluid and called
the flow direction
positive.
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Law of Charges
• Like charges repel
• Opposite charges attract
The Electron
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Sir William Crookes (1855) invented the
Crookes tube or cathode ray tube (CRT)
Crookes could actually see some sort of particle
beam going from the negative cathode to the
positive anode.
It wasn’t until 1897 that this “tube” was used to
change Dalton’s Billiard Ball Model.
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Joseph John Thomson (England 1897)
• J.J. Thomson took the
Crookes tube and
experimented with it. He
held a magnet to the side of
the tube and tried to direct
the beam of light.
• He discovered the electron
while experimenting with
cathode rays.
Thomson took Crookes’ tube and used a magnet to bend the ray.
The ray bent towards the positive side of the magnet. He
concluded that the ray must have had a negative charge if it was
attracted to the positive side. These negative charges became
known as electrons
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Thomson’s Cathode Ray Tube
http://www.youtube.com/v/GzMh4q-2HjM
Thomson Experiment
https://sites.google.com/site/physicsflash/home/thomson
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Thomson’s Discovery
He determined that the cathode ray was made of
negatively charged particles – electrons
Thompson showed that the production of the
cathode ray was not dependent on the type of gas
in the tube, or the type of metal used for the
electrodes.
His discovery of the electron won the Nobel Prize in
1906
The Plum Pudding Model
Thomson did not know how
the electrons in an atom
were arranged. He believed
they were mixed throughout
an atom.
He proposed that the atom
was a sphere of positively
charged material. Spread
throughout the atom were
the negatively charged
electrons similar to plums in a
pudding or chocolate chips in
ice cream.
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Robert Millikan - 1909 determined the size of the charge on
an electron. He also determined that there was a smallest
'unit' charge, or that charge is 'quantized'. He received the
Nobel Prize for his work.
http://www.youtube.com/v/XMfYHag7Liw
Electrical Charge
• Charge of Electron
1.6 x 10-19 C (coulombs)
• Rest Mass of Electron
9.11 x 10-28 g
• Electric charges always exist in whole
number multiples of a single basic unit,
the electron
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The Proton and Atomic Nucleus
Eugen Goldstein (German, 1886)
Observed that cathode-ray tubes with a
perforated cathode emit a glow from the end of
the tube near the cathode.
Concluded that there is another ray
(positive particle) that travels in the
opposite direction (opposite the
cathode ray), from the anode
toward the cathode. Because these
rays pass through the holes, or
channels, in the cathode, he called
them canal rays.
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Canal Rays passed through holes, or
channels, in the reverse direction as the
cathode ray
https://www.youtube.com/v/3WIjCtZLMDg
Canal Rays
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Ernest Rutherford (England, 1911)
• Tested Thomson’s
theory of atomic
structure with the “gold
foil” experiment in
1910.
• Proposed the name
proton for the positively
charged particles in the
nucleus of an atom
Gold Foil Experiment
• Bombarded thin gold foil with a beam of
‘alpha’ particles (Helium nucleus).
• If the positive charge was evenly spread
out, the beam should have easily passed
through.
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Rutherford's Gold Foil Experiment
http://www.youtube.com/v/5pZj0u_XMbc
Targets in the Clouds... Give it a try
-Find target hidden by a black cloud
-Shoot beams into the cloud
-Record where the beams come out
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Targets in the Clouds Revealed
the first target was:
a wedge
the second target was:
a circle
Expected
Found
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Rutherford's Experiment
Most particles passed through with no deflection,
while some were highly deflected
Rutherford concluded that most particles passed
through because the atom is mostly empty space.
Rutherford’s Conclusions
• All of the positive charge, and most of the
mass of an atom are concentrated in a
small, dense core called the nucleus.
• Atoms are mostly empty space.
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Rutherford’s Nuclear Model
The Neutron
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• 1932, James Chadwick
proved the existence of
the neutron
• Chadwick bombarded
beryllium with alpha
particles.
• The beryllium emitted a
neutral particle which in
turn was used to
bombarded the hydrogen
atoms in paraffin (and
other elements as
targets).
Chadwick’s Apparatus
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• Determine that the neutron did exist and
that its mass was about 0.1 percent more
than the proton's.
• No electrical charge.
• His work lead the way to neutron
bombardment of the uranium atom,
splitting its nucleus and releasing the huge
amounts of energy.
Subatomic Particles – Fundamental Constants
Particle
Symbol Relative
Charge
Mass
Relative to
the Proton
Rest Mass (g)
Electron
e-
1-
1/1840
9.11 x 10-28 g
Proton
p+
1+
1
1.67 x 10-24g
Neutron
n0
0
1
1.67 x 10-24g
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http://www.pbs.org/wgbh/aso/databank/entries/dp32ne.html
http://www.vzhang.com/vzfiles/joseph_proust.htm
http://plato.stanford.edu/entries/democritus/
http://chemed.chem.wisc.edu/chempaths/GenChem-Textbook/TheAtomic-Theory-687.html
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