
Bohr1922 
He mathematical worked out a model of a hydrogen atom. This explained the emission line spectrum of only certain wavelengths
(certain colours) using Planck's quantum concept. The Bohr atom Model: electron energy levels are fixed at certain amounts
or distances from nucleus; electron transitions from higher to lower levels result in separate wavelengths of emitted light.
Bohr conjectured that an atom could exist only in a discrete set of stable energy states. 
Here we see Bohr's model of the atom.

Bohr also contributed to the clarification of the problems encountered in quantum physics
Chemirty 11 project The Evolution of the Atomic Theory 