When electrons that have been excited by gaining energy return to a lower energy level in an atom they emit photons and produce an emission spectrum.

Evidence for the existence of electrons in discrete energy levels comes from the line emission spectrum of hydrogen, in which the lines converge at higher energies.

The main energy level (or shell) is identified by an integer number, n, and can hold a maximum number of electrons equal to 2n2.

The main energy levels can be split into s, p, d and f sub-levels with successively higher energies.

Sub-levels contain a fixed number of orbitals. An orbital is a volume of space where there is a high probability of finding an electron.

Each orbital has a defined energy state for a given electronic configuration and chemical environment.

Each orbital can contain a maximum of two electrons each with opposite spin.