Hund’s Rule : Electron pairing in orbitals starts only when all available empty orbitals of the same energy (degenerate orbitals) are singly occupied.
Ex: Carbon atom.
The configuration of carbon © atom (Z = 6), is ${{1s}^{2}}$ ${{2s}^{2}}$ ${{2p}^{2}}$. The first four electrons go into the "1s and 2s orbital. The next two electrons go into separate 2p
orbitals, with both electrons having the same spin according to Hund’s rule.