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.
