The atomic theory has evolved over time from early Greek philosophers' idea that all matter is made of indivisible atoms, to modern scientific discoveries about atomic structure. Democritus first proposed the idea of atoms in 460 BC. In the early 1900s, Rutherford discovered the nucleus through his gold foil experiment, and Bohr proposed that electrons orbit in specific energy levels. Chadwick discovered the neutron in the 1920s, completing the standard model of the atom's subatomic particles. Today's electron cloud model depicts electrons as existing in regions rather than fixed orbits.

1. The Evolution of The
Atomic Theory

2. Democritus
Lived: 460BC-370BC
Main concepts:
Everything is made of atoms
Atoms are indivisible (cannot be
divided)
Atoms are small and solid particles
Named the atom (called it “atomos”)

3. Dalton
Lived 1766-1844
Main concepts:
Each element has a different atom
and a different atomic weight
All atoms of the same element are
perfectly identical
Atoms cannot be created or
destroyed in chemical reactions.

4. Thomson
Lived: 1856-1940
Main concepts:
Discovered that atoms ARE divisible
Discovered the electron through his
Cathode-Ray experiment
Atoms have a much larger mass than
electrons
Inferred that atoms have a positive charge
Plum Pudding Model

5. Rutherford
Lived 1871-1937
Discovered that there is a small, dense
positively charged nucleus
Discovered that most of an atoms mass
is in it’s nucleus
Predicted the existence of neutrons
Stated that electrons move rapidly in
the empty space around the nucleus
Conducted the gold foil experiment
Stated that atoms are mostly empty
space.

6. Bohr
Lived 1885-1962
Stated that electrons can only travel at
specific distances from the nucleus
Stated that electrons can jump between
levels/paths around the nucleus
Stated that the chemical properties of
an element are determined by how
many electrons are in the outer shell.

7. Chadwick
Lived 1891-1974
Discovered the previously unknown part
of the nucleus of an atom- the Neutron!
Named the neutron because of it’s lack
of an electrical charge.
His discovery made it possible to create
new elements in the laboratory (heavier
than Uranium) which was not possible
before.

8. Electron Cloud Model
This is our most modern and
accepted model of the atom
It differs from Bohr’s model
because it states that the electrons
can be found in a general region
around the nucleus, not specific
paths.

9. Time for:
The atom song!
http://www.youtube.com/wat
ch?v=vUzTQWn-wfE