This page shows the electron configurations of the neutral gaseous atoms in their ground states.
In many cases, multiple configurations are within a small range of energies and the irregularities shown below do not necessarily have a clear relation to chemical behaviour.
[2] All sources concur with the data above except in the instances listed separately: This website is also cited in the CRC Handbook as source of Section 1, subsection Electron Configuration of Neutral Atoms in the Ground State.
Hoffman, Darleane C.; Lee, Diana M.; Pershina, Valeria (2006).
In Morss; Edelstein, Norman M.; Fuger, Jean (eds.).