Built upon the Rutherford model of the atom
Said that excited, low pressure gases give off their own bright-line spectrum
Said electrons only radiate energy in the form of light when they change orbits
An electron can not orbit at just any radius around the nucleus, but only at certain quantized orbits
Electrons can change orbits when they absorb or emit energy (if gets exactly enough energy to jump one level, jumps one level. If gets enough, can jump multiple levels. If doesn't get enough energy, ignores it)
Once an electron absorbs enough energy to lift it to the highest energy level, it can escape from the atom altogether and any excess energy is converted to kinetic energy (atom is ionized)
When an electron is in a higher energy level, it can jump down to a lower energy level by releasing energy in the form of a photon of light (energy of photon is equal to the difference between the energy levels the electron moves between)