Hund's rule of maximum multiplicity

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

                                                                   Electrons

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 (p_x, p_y, p_z) orbitals. The fourth electron entering the p- level will go to p_x 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.