The
electron configuration for an atom indicates the
occupied subshells for that atom. For an atom in the ground state the available electrons occupy subshells of
lowest possible energy.
Two additional guidelines based on experimental observations are required to predict the electron configuration:
- The Pauli Exclusion Principle:
No two electrons in the same atom can have the same four quantum numbers.
Recall that electrons in the same orbital have the same values of n, l and ml. As there are only two allowed values of ms, this limits the number of electrons per orbital to two. Thus in the- s-subshell – one orbital therefore two electrons maximum
- p-subshell – three orbitals therefore six electrons maximum
- d-subshell – five orbitals therefore ten electrons maximum
- Hunds Rule:
Electrons occupy orbitals of equal energy singly before a second electron enters any of them. Recall that all of the orbitals in a subshell are of equal energy
Hence a p-subshell with four electrons has two unpaired.