Electron configurations are governed by a limitation called as the Pauli Exclusion Principle:

• No two electrons can have the same value for all four quantum numbers n, l, m and ms. An alternative statement is
• A maximum of two electrons is possible in any orbital. Thus the three-electron lithium atom could not have the electron configuration (1s)3; the ground state is (1s)2(2s)1. When p, d,...orbitals are filled it is important to remember that 3, 5,...m values are possible. A set of p orbitals with any n can be filled by a maximum of six electrons and a set of d orbitals by 10. THE PERIODIC TABLE