Principles of distributing electrons

Principles of distributing electrons:
Auf-beau principle: electrons occupy the sublevels of lower energy first then of higher one
This diagram predicts the following order of increasing energy for atomic orbitals

1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s < 4d < 5p < 6s < 4f < 5d < 6p < 7s < 5f < 6d < 7p < 8s...

Hound's rule: No electron pairing takes place in a given sub level until each orbital contains 1 electron

Note:

(1) Electron configuration of Chromium are [Ar] 4s1 3d5 not [Ar] 4s2 3d4 because when the orbit is half filled the element becomes more stable.
(2) Electron configuration of Cupper are [Ar] 4s1 3d10 not [Ar] 4s2 3d9 because when the orbit is completely filled the element becomes more stable.