What the scientists did in chronological order - KEY.
Development of the ATOMIC THEORY TIMELINE
Democritus
465 BC
Greek philosopher
Hypothesized that atoms cannot be destroyed, are always
moving, and are invisible
John Dalton
1803
Created the first real atomic theory
Hypothesized that the atom was a tiny hard sphere
Believed that atoms of a given element are identical
Dmitri Mendeleev
1869
Suggested future elements would be discovered and
fitted into the periodic table
Proposed the basis of our modern periodic table
JJ Thomson
1897
Discovered the electron
Used the cathode ray tube in his discovery
Created the “plum pudding” model of the atom
Ernest Rutherford
1911
Discovered that the atom is mostly empty space
Discovered that the proton had a positive charge
Did the gold foil experiment
Niels Bohr
1915
Created a model of the atom with electrons moving
around the nucleus in fixed orbits
Werner Heisenberg
1925
Uncertainty principle: A particle’s position, energy, and
time can never be precisely known
Worked with the Quantum Theory of the Atomic Model
James Chadwick
1932
Discovered the neutron
Found the missing mass of the atom’s nucleus
