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History of Atomic
Theories
(The Changing Views of Atomic Structure)
Empedocles
(Greek Philosopher, 450 B.C.)
Everything that existed was thought to
be composed of four elements.
(i) Water
(iii) Air
(ii) Fire
(iv) Earth
Democritus
(Greek Philosopher, 400 B.C.)
All matter is made of tiny hard
indestructible particles which
are indivisible (i.e. can not be
broken down). The Greek word
for “indivisible” is atomos,
therefore all matter is made up
of atoms.
Aristotle
(Greek Philosopher, 350 B.C.)
 Believed in Empedocles’
“four element” model.
-Despite the more recent
“atomic model” proposed by
Democritus, Aristotle was so
influential that the “four
element” model was accepted
for almost 2000 years.
Note: The Greek philosophers
did not test their ideas with
experiments. They were thought
of as great “thinkers”, but not
scientists.
Alchemists
(Philosophers, Mystics, Magicians,
Chemists, 500-1600 A.D.)
Performed experiments and
devised chemical symbols for
substances that we now call
elements and compounds.
They also invented many lab tools
that are still used today…
John Dalton
(English Scientist, 1800’s)
Using experiments, he discovered that
1. Atoms are tiny indestructible particles that
cannot be broken down
2. Atoms combine with other atoms to form
molecules (e.g. carbon + oxygen  carbon dioxide)
3. Atoms of an element are identical (e.g. H2 gas
are all H atoms)
4. Atoms do not lose their identity during chemical
reactions (i.e. cannot be created and/or destroyed)
5. Molecules of a compound are identical
(e.g. water = H2O)
Dalton developed a 6 part theory
based on these experiments which
explains
1. The Law of Conservation of Mass
 The total mass of the reactants = the
total mass of the products
2. The Law of Constant Composition
 A compound always contains the same
elements in the same proportions by
mass.
Model: Billiard Balls
Theory
Model
Analogy
(Featureless
Sphere)
(Billiard Balls)
 Indivisible atoms
J.J. Thompson
(Physicist, 1904)
Thompson discovered electrical
particles can be lost or gained
from elements. He named these
particles electrons. He also
believed that atoms were made of
positively charged matter with
negatively charged electrons
scattered throughout.
Model: The Raisin Bun Model
Theory
Model
Analogy
(Uniform Charge
Distribution)
(Raisin Bun)
 Electrons
embedded within a
positive sphere
 Net charge of zero
Hantaro Nagaoka
(Japanese Scientist, 1904)
He modeled the atom as a large
positive sphere surrounded by a
ring of negative electrons.
Model: The Saturn Model
Theory
Model
Analogy
(Ring of Flat
Electrons)
(Saturn)
 Positive sphere
with ring of
electrons
Ernest Rutherford
(Nuclear Physicist, 1911)
At McGill University (in
Montreal), Rutherford designed
the “Gold Foil Experiment” to
test Thompson’s and Nagaoka’s
models
The Gold Foil Experiment
 Positive alpha particles were fired at a
thin sheet of gold foil
 Rutherford predicted that the alpha
particles would pass directly through the
metal foil, untouched.
The Gold Foil Experiment
The Gold Foil Experiment
The Results…
 Most alpha particles passed through in
a direct path, however, some deflected and
bounced back.
“Its like shooting a bullet at a
piece of tissue paper and
having the bullet bounce off”
The Gold Foil Experiment
The Conclusion…
 Atoms are mostly empty space with a tiny
central nucleus which contains almost all of
the total mass of the atom and is positively
charged. He called these positively charged
particles protons. Protons have a mass nearly
200x greater than electrons.
 The nucleus is surrounded mostly by empty
space, containing rapidly moving negative
charges called electrons.
Model: The Nuclear Model
Theory
Model
Analogy
(Nuclear Model)
(Beehive)
 Small positive
nucleus
surrounded by
negative electrons
James Chadwick
(Nuclear Physicist, 1932)
Chadwick discovered particles in
the nucleus of an atom possessing
no electrical charge (neutral). He
called them neutrons. The
neutrons make up the remaining
mass of the nucleus.
Neils Bohr
(Danish Physicist, 1920)
Suggested electrons moved
around the nucleus in a definite
“orbit” arranged in “shells”.
Model: The Planetary Model
Theory
Model
Analogy
(Planetary
Model)
(Planets Orbiting
Around the Sun)
 Explains periodic
law
 Electrons are
quantized in energy
 levels
Neutrons + Protons are
in the nucleus
Electrons in orbits
around the nucleus
Protons = Positive Charge
Electrons = Negative Charge
Neutrons = Zero Charge
HOMEWORK
 Read pg. 82 – 85
 Answer # 1 – 6, 9 on pg. 85
 Read pg. 90 – 92
 Answer # 1 – 3 on pg. 93
The End