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.
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 −1)d1−10 ns1−2, Here, n = 4 to 7 . In periods 6 and 7, (except La and Ac) the configuration includes ( (n −2) f orbital ; [Noble gas] ( n −2) f 14 ( n −1)d1−10 ns1−2 .
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