What elements has a ground-state electron configuration different from the predicted one?

Answer 1

There are many elements among the transitional elements which has the electron configuration differed than predicted. Such element with the lowest atomic number is chromium.

Answer 2

Transition metals have ground-state electron configurations that differ from the predicted ones due to the exchange of electrons between the ns and (n-1)d subshells. This exchange stabilizes the d orbitals, leading to configurations that are closer to half-filled or fully filled d subshells. Examples include chromium ([Ar] 3d^5 4s^1) and copper ([Ar] 3d^10 4s^1).