1. Models of the atom
The Rutherford Model investigated the scattering of alpha particles by thin gold foil.
Alpha particles are positively charged helium atoms which are produced as a decay from a radioactive substance.
Why was gold foil used? Because, you can pound gold so thin that it is only a couple of atoms thick.

2. Rutherford Continued
The scattering experiment showed us the following:
The atom is comprised of mostly empty space with the nucleus as a very small dense positively charged core.
The paths of the deflected alpha particles were in hyperbolic paths this shows that Coulombs forces caused the “+” alpha particle to be repelled by the “+” nucleus.

3. The Bohr Model
The Bohr model said the nucleus was made up of protons and neutrons.
The electrons orbited the nucleus and would shift between energy levels.
The Bohr model was better than Rutherford’s model because it accounted for the emission of spectrums and the unique spectrum of each element.

4. Standing waves for electrons can exist only at certain distances from the nucleus, which correspond to the discrete energy levels of the atom.
Bohr’ model could not predict other aspects of the hydrogen atom nor can it explain the electron orbit of larger atoms having many electrons.

5. Cloud Model
In the cloud model of the atom, there is no specific orbit for an electron as it moves about the nucleus. Instead there is a region of most probable electron location called a state.
A state can hold no more than two electrons.
Werner Heisenberg came up with the Uncertainty Principle which said you can not locate an electron with out knowing its momentum.