Electron shells are regions around an atomic nucleus that represent discrete energy levels where electrons are likely to be found. These shells are defined by quantum mechanics and are associated with a specific principal quantum number ().

Subshell

Each shell is composed of one or more subshells, which themselves are composed of one or more orbitals. For example, the first shell has one subshell, called 1s, the second shell has two subshells, called 2s and 2p.

Subshells are named by their subshell label which originate from their historic names and the shell they are contained in.

The various possible subshells are shown in the following table:

	Subshell Label	Max Electrons	Shells Containing It
0	s	2	Every shell
1	p	6	2nd shell and higher
2	d	10	3rd shell and higher
3	f	14	4th shell and higher

Historical Names

	Historical Name
0	sharp
1	principal
2	diffuse
3	fundamental

Subshell Energies

Every electron in a subshell have exactly the same amount of energy, with later subshells having higher energies than earlier ones.

Hydrogen-Like Atoms

For hydrogen-like atoms the energy for all subshells in a shell are the same and are given by the following equation:

Where is the subshell energy for subshells in the shell and is the atomic number of the atom.

Multiple Electron Systems

For non-hydrogen-like atoms the energies for a given subshell in a shell can be unique and are given by the following equation:

Where is the subshell energy for the subshell with the quantum numbers and .

Filling Order

Subshells are filled in a specific order to follow the Aufbau principle, which states that electrons fill the lowest energy level subshell available first, before they begin to fill higher energy subshells. This filling follows the rule which states that subshells with a lower value are filled before ones with a higher value.