We have already learnt in XI STD to write the electronic configuration of the elements

using Aufbau principle, Hund’s rule etc. According to Aufbau principle, the electron first fills the 4s orbital before 3d orbital. Therefore filling of 3d orbital starts from Sc, its electronic configuration is [Ar]3d1 4s2 and the electrons of successive elements are progressively filled in 3d orbital and the filling of 3d orbital is complete in Zinc, whose electronic configuration is [Ar] 3d10 4s2. However, there are two exceptions in the above mentioned progressive filling of 3d orbitals; if there is a chance of acquiring half filled or fully filled 3d sub shell, it is given priority as they are the stable configuration, for example Cr and Cu.

The electronic configurations of Cr and Cu are [Ar] 3d5 4s1 and [Ar] 3d10 4s1 respectively. The extra stability of half filled and fully filled d orbitals, as already explained in XI STD, is due to symmetrical distribution of electrons and exchange energy.

Note: The extra stability due to symmetrical distribution can also be visualized as follows. When the d orbitals are considered together, they will constitute a sphere. So the half filled and fully filled configuration leads to complete symmetrical distribution of electron density. On the other hand, an unsymmetrical distribution of electron density as in the case of partially filled configuration will result in building up of a potential difference. To decrease this and to achieve a tension free state with lower energy, a symmetrical distribution is preferred.

With these two exceptions and minor variation in certain individual cases, the general electronic configuration of d- block elements can be written as [ ]Noble gas n d ns−( ) − −1 1 10 1 2, Here, n = 4 to 7 . In periods 6 and 7, (except La and Ac) the configuration includes

(_n f_−( )2 orbital ; [ ]Noble gas n 2 f n d ns14−( ) −( ) − −1 1 10 1 2 .