shift results in all orbitals of the subshell of higher energy getting either completely filled or half filled. The valence electronic configurations of Cr and Cu, therefore, are d s and d s respectively and not d s and d s . It has been found that there is extra stability associated with these electronic configurations. The completely filled and completely half-filled subshells are stable due to the following reasons: .
Symmetrical distribution of electrons: It is well known that symmetry leads to stability. The completely filled or half filled subshells have symmetrical distribution of electrons in them and are therefore more stable. Electrons in the same subshell (here 3d ) have equal energy but different spatial distribution. Consequently, their shielding of one- another is relatively small and the electrons are more strongly attracted by the nucleus.
. Exchange Energy : The stabilizing effect arises whenever two or more electrons with the same spin are present in the degenerate orbitals of a subshell. These electrons tend to exchange their positions and the energy released due to this exchange is called exchange energy. The number of exchanges that can take place is maximum when the subshell is either half filled or completely filled (Fig.
. ). As a result the exchange energy is maximum and so is the stability. You may note that the exchange energy is at the basis of Hund’s rule that electrons which enter orbitals of equal energy have parallel spins as far as possible.
In other words, the extra stability of half-filled and completely filled subshell is due to: (i) relatively small shielding, (ii) smaller coulombic repulsion energy, and (iii) larger exchange energy. Details about the exchange energy will be dealt with in higher classes. Fig. .
Possible exchange for a d configuration Causes of Stability of Completely Filled and Half-filled Subshells Table . Electronic Configurations of the Elements * Elements with exceptional electronic configurations ** Elements with atomic number and above have been reported but not