Pauli Exclusion Principle

Electron configurations are governed through a limitation known as the Pauli Exclusion Principle:

• No two electrons can have similar values for all four quantum numbers n, l, m and ms.

A different statement is

• In any orbital a maximum of two electrons is possible.

So the three-electron lithium atom cannot have the electron configuration (1s)3; the ground state is (1s)2(2s)1. When p, d,...orbitals are occupied it is significant to remember that 3, 5,...m values are possible. A set of the p orbitals with any n can be taken by a maximum of six electrons and a set of d orbitals by 10.