Explain Hund's rule with an example .

Explanation

Hund's Rule : According to this rule electron pairing in orbitals starts only when all available empty orbitals of the same energy are singly occupied .
(1) Ex :
1) The configurationof carbon atom (Z = 6) is `1s^(2) 2s^(2) 2p^(2)` .
2) The first four electrons go into the 1s and 2s orbitals .
3) The next two electrons go into `2p_(x)` and `2p _(y)` orbitals .

4) But , they do not pair in `2p_(x)` orbital.
(2) Ex :
1) In oxygen atom `(""_(8) O)` , distribution of electrons is given below .
2) `""_(8) O`

3) Here , pairing of electrons in `2p_(x)` orbital takes place after , `2p_(x) , 2p_(y)` and `2p_(z)` orbitals are filled with a single electron .