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.
The Pauli Exclusion Principle is an additional guideline based on experimental observations.
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 the
- s-subshell has one orbital therefore two electrons maximum
- p-subshell has three orbitals therefore six electrons maximum
- d-subshell has five orbitals therefore ten electrons maximum
Electron configurations for atoms deduced using these guidelines are consistent with the position of the atom in the periodic table.