* Electrons orbit the nucleus in fixed circular paths called shells.

* Electrons can only gain or lose energy by jumping from one shell to another.

* Each shell has a specific energy level. The closer the shell is to the nucleus, the lower the energy level.

* Quantum-mechanical model - based on quantum principles

Plum-Pudding Model

* Electrons are embedded within a positively charged "pudding" of matter.

* There was an absence of energy considerations within energy levels

* Electrons could be anywhere within the atom.

* Classical- mechanical model- no consideration quantum properties of matter