Confused about Copper and Chromium [ENDORSED]

[JT Wechsler 2B]

Today in class, Professor Lavelle was saying that the electron configurations for the elements copper and chromium are abnormal and will take an electron from the 4s level to fill some of the 3d level. Does this have something to do with the energy levels of the elements?

[Christopher Tran 1J]

Essentially chromium has five 3d orbitals, four of which have a valence electron. Another electron is taken from the 4s energy level in order to occupy the fifth 3d orbital (so that all five 3d orbitals have one electron each) since symmetry is favored.
The same happens for copper, where all but the last 3d orbital have a pair of electrons each. An electron is taken from the 4s level to occupy the last 3d orbital and maintain symmetry.

[Erin Nash - 4G]

The reason that the configuration is 3d5 4s1 and not the expected 3d4 4s2, if because in the d shell, there are 5 possible pairs of electrons. According to Hund's principle, an electron must spin parallel, until there are no free orbitals left. Thus, an atom would prefer to have all 5 orbitals filled in the d shell, so it takes an electron from the 4s shell to achieve this.