Chemistry question!?

Question

Why is the Predicted ground state and Observed ground state of some elements different?

For example, The PGS of Chromium is: 1s2 2s2 2p6 3s2 3p6 4s2 3d1 3d1 3d1 3d1 3d0.

On the other hand, the OGS is 1s2 2s2 2p6 3s2 3p6 4s1 3d1 3d1 3d1 3d1 3d1.

The PGS has 4s2 and 3d0, while the OGS has 4s1 and 3d1.

Why is that?

Answer 1

The usual answer to this question is that there is an extra stability associated with a half-filled d sub-level, so having 5 is more stable than 4. So an electron is "borrowed" from 4s, to give 3d5 and 4s1.