# Exceptions to the Aufbau principle?

Why are there exceptions to the Aufbau Principle? How do the abnormalities occur and why? If you could specify a few exceptions that would be swell.

• – Qmechanic Nov 20 '16 at 17:52

Some exceptions occur in the Lanthanide group of elements (aka 'Rare Earths', aka first $\mathrm{f}$ block group). These should have the electron configuration:
$$[\mathrm{Xe}]4f^n6s^2$$
But due to the energy similarity between the $4f$ and $5d$ orbitals, 'exceptions' so arise. Lanthanum (symbol $\mathrm{La}$, $Z=57$) is one of these. Its electron configuration is:
$$[\mathrm{Xe}]5d^16s^2$$
But these exceptions don't generally have serious consequences: despite the 'anomalies' the Lanthanides still show the $\mathrm{+3}$ oxidation state in most of their chemical compounds.