Hund's rule of maximum multiplicity
Hund's rule of maximum multiplicity states, that in filling p, d or f orbitals, as many unpaired electrons as possible are placed before pairing of electrons with opposite spin is allowed. Pairing of electrons requires energy. Therefore no pairing occurs until all orbitals of a given sub-level are half filled. This is known as Hund's rule of maximum multiplicity. It states that when electrons enter sub-levels of fixed (n+1) values , available orbitals are singly occupied (Table).
Atomic Number of
Number Element 1s 2s 2px 2py 2pz Unpaired
1 H up 1
2 He both 0
3 Li both up 1
4 Be both both 0
5 B both both up up 1
6 C both both up up 2
7 N both both up up up 3
8 O both both both up up 2
9 F both both both both up 1
10 Ne both both both both both 0
Thus, if three electrons are to be filled in the p- level of any shell, one each will go into each of the three (px, py, pz) orbitals. The fourth electron entering the p- level will go to px orbital which now will have two electrons with opposite spins (as shown above) and said to be paired. The unpaired electrons play an important part in the formation of bonds.