3.1 Early Atomic
Theories and the
Origins of Quantum
Theory
Democritus
Aristotle
Ancient Philosophy
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Who: Aristotle, Democritus
When: More than 2000 years ago
Where: Greece
What: Aristotle believed in 4 elements: Earth,
Air, Fire, and Water. Democritus believed
that matter was made of small particles he
named “atoms”. They debated in public
forums in front of a crowd. Aristotle was
much more charismatic and forceful and won
the arguments.
Alchemists
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Who: European Scientists
When: 800 – 900 years ago
Where: Europe
What: Their work developed into what is now
modern chemistry.
• Why: Trying to change ordinary materials
into gold. Lead to the concept of
change…lead to the development of
glassware like Erlenmeyer flasks, beakers…
Alchemic Symbols
• Proust is responsible for the law of definite
proportions. It states that the elements
combined to form compounds in certain welldefined proportions, rather than mixing in just
any proportion.
Lavoisier (husband and wife team)
proposed the Law of conservation of
mass. Matter is never created or
destroyed in a chemical reaction
John Dalton
Particle Theory
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Who: John Dalton (high school teacher)
When: 1808
Where: England
What: Described atoms as tiny particles that
could not be divided. Thought each element
was made of its own kind of atom. Basic
ideas of his theory are still correct. Is credited
with the law of multiple proportions.
• If two elements form more than one compound between
them, then the ratios of the masses of the second element
which combine with a fixed mass of the first element will be
ratios of small whole numbers
J. J. Thompson
Discovery of Electrons
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Who: J. J. Thompson
When: 1897
Where: England
What: Thompson discovered that electrons
were smaller particles of an atom and were
negatively charged.
• Why: Thompson knew atoms were neutrally
charged, but couldn’t find the positive
particle. Used cathode ray tubes for his
experiments.
Cathode rays tubes
J.J. Thomson studied was the conduction of electricity through
gases.
One subject which interested Thomson was cathode rays.
These rays are emitted at the cathode, or negative terminal, in
a discharge tube. I
The nature of the cathode rays was controversial. Although
Thomson thought the rays must be particles, many Europeans
thought they were an 'etherial disturbance', like light. In
Germany Hertz had observed the rays passing through thin
sheets of gold. It seemed impossible that particles could pass
through solid matter.
Cathode rays being deflected by a
magnetic field, whose direction is
into the paper.
The magnetic field produces a
force which deflects the cathode
rays. In his tube, Thomson
positioned the coils so that the
deflection was in the opposite
direction to the deflection produced
by the electric field.
By adjusting the strengths of the
fields the rays could be deflected,
in one direction by the electric
field, and back an equal amount by
the magnetic field. The forces were
balanced.
Ernest Rutherford
Atomic Structure I
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Who: Ernest Rutherford
When: 1911
Where: England
What: Conducted an experiment to isolate
the positive particles in an atom. Decided
that the atoms were mostly empty space, but
had a dense central core.
• Why: He knew that atoms had positive and
negative particles, but could not decide how
they were arranged.
Gold foil experiment
He fired Alpha particles at an
extremely thin gold foil.
• He expected them to go straight
through with perhaps a minor
deflection.
• Most did go straight through, but
to his surprise some particles
bounced directly off the gold
sheet!
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• What did this mean?
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• Rutherford hypothesized that the
positive alpha particles had hit a
concentrated mass of positive
particles, which he termed the
nucleus.
Rutherford’s Lab at McGill
Atomic Structure II
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Who: Niels Bohr
When: 1913
Where: England
What: Proposed that electrons traveled in
fixed paths around the nucleus. Scientists
still use the Bohr model to show the number
of electrons in each orbit around the nucleus.
• Why: Bohr was trying to show why the
negative electrons were not sucked into the
nucleus of the atom.(which are +)
Electron Cloud Model
• Electrons travel around the nucleus in
random orbits.
• Scientists cannot predict where they will be
at any given moment.
• Electrons travel so fast, they appear to form
a “cloud” around the nucleus.
Electron Cloud Model
• Neil’s Bohr Experiment…
• Studied light emitted from a
heated source after it is
seperated by a prism.
• Light is SPECIFIC and has
specfic energy (colour)…a
full rainbow is never seen.
Thus electrons have
specific energy…”orbit” is
defined
• Based on work by Planck –
light is emitted in discreet
quanta
Neils Bohr
• Rays or Particles? Marie Curie
• Curie-Joliots in Paris, had bombarded
beryllium with alpha-particles, and found that
very penetrating gamma rays were emitted.
Cockcroft and Walton thought that similar rays
should be observed when they bombarded
light elements with protons.
• Unfortunately, the Curie-Joliots were mistaken
in identifying the 'radiation'. In January 1932
James Chadwick repeated their experiments
and found that the reaction produced neutral
particles, neutrons, that Rutherford had
predicted in 1920. A few months after
Chadwick's discovery Rutherford came to
Cockcroft and Walton and told them they
'ought to put in a fluorescent screen and get
on with the job'. Rutherford was clearly hoping
to see alpha-particles, and a fluorescent
screen would be the best way to detect them.
• Led to the idea that the nucleus is
BREAKABLE…new atoms can be created!!!
Led to the development of the nuclear bomb
with uranium (“little boy”)
On 14 April 1932 Walton set up the tube
and bombarded lithium with high energy
protons. He then crawled into the little
observation cabin set up under the
apparatus and immediately saw
scintillations of the fluorescent screen. The
reaction was giving off alphaparticles…which are HELIUM NUCLEI!
Led to the development of the nuclear
bomb with plutonium (“Fat man” 20 000
TNT) and atomic bombs with hydrogen. ( 1
million TNT)