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History of the Atomic Model
Seeing the Invisible
A Big Debate
Can matter be divided into
smaller and smaller pieces
forever?
YES!
NO!
Atomic Models: Greek
Democritus (460-370 B.C.)
 Matter can not be
divided forever
• Smallest piece = “atom”
(Gk “atomos” = “not to be cut”)
• He had no way of
knowing what atoms
looked like!
The word “atom” comes from a Greek
word that means “unable to be cut”
… and you
Imagine
kept on cutting
you
had a
the leftover
piece
piece of
in half…
gold that
you then
cut in
half…
Go
ld
Gold
…and then you
cut one of
these smaller
pieces in half…
The word “atom” comes from a Greek
word that means “unable to be cut”
…and kept
…and
kept
going…
going…
An atom of gold
Eventually you would have
…and
1 piece of gold
left. kept
If you
going…
cut it in half, you
wouldn’t
have gold any more –
you’d have something
else. This tiny, tiny single
piece of gold is called an
atom of gold. An atom is
the smallest particle of an
element that acts like the
element.
Democritus
He hypothesized that atoms were:
– Small & Hard
– Different in shape & size
– Infinite
– Always moving
– Capable of joining
Time Goes By…
• 1600s-1700s: Key experiments occur
which support Democritus’s ideas.
• Robert Boyle (1627-1691)
• Antoine Lavoisier (1743-1794)
Dalton (1766-1844)
• English chemist and school
teacher
• Did many experiments, studying
gasses and proposed atomic
theory
Atomic Models: Dalton
1. Elements composed of atoms; atoms are
indestructible
2. Atoms of the same element are exactly
alike
3. Atoms of different elements are different
4. Compounds formed by joining 2 atoms
Atomic Models: J.J. Thompson
• Passed electricity
through an uncharged
gas
– The gas gave off rays to
show it was
NEGATIVELY charged
– How?
– Negative charges must
come from inside the
atom!
=> Electrons!
But wait…
• How can an atom be
NEUTRAL if it is full of
negatively charged particles
(electrons?)
Atomic Models: J.J. Thompson
• The atoms are
neutral… How?
• + charges must be
present to balance charges
• + & - lumped in a
cluster he said looked
like “plum pudding”
Thompson’s Model
Atomic Models: Rutherford
• Passed + Charged Particles through
gold foil
– Most passed right through
• Atom is mostly empty space
– Some bounced off at odd angles
• Nucleus must be +
• Calculate size of nucleus
Rutherford’s Gold Foil
Experiment
Rutherford’s Work
Rutherford’s Model
Okay…
• So the atom is made up of
positive and negative
particles.
• Where are the electrons
found in the atom?
Atomic Models: Bohr
• Electrons are
found in specific
energy levels
– Like planets
around the sun
Bohr’s Model
Atomic Models: Wave
• Electrons move so fast that it is
impossible to determine their
location
• Move in all directions around
the nucleus