[عاطف خليفة]

Principals of distributing electrons

* I) Auf-bau (building - up) principle.

1. Electrons occupy the orbitals in the order of increasing orbital energy the lowest energy orbitals are filled first

2. The sequence of energy sublevels as follows :-
1s < 2s < 2p< 3s < 3 p < 4S < 3d < 4p < 5S < 4d
< 5p < 6s <4f < 5d < …

II) Hund’s rule

• No electron pairing takes place in a given sublevel until each orbital contains one electron

• The spin of single electrons is in the same direction

• It is preferable for an electron to be paired with another one in a lower energy level than to jump to a higher energy level

Ex
N=7 , O = 8The distribution may be:
1) 2,5 according to principal levels
2) 1 s2 , 2s2 , 2p3 according to Auf-bau principal.
3) 1 s2, 2s2 , 2Px1 , 2Py1 2Pz1 according to Hund’s law.