In the transition elements, d-orbitals of penultimate shell are progressively filled. There are mainly three transition series. The first transition series involves me filling of 3d-orbitals. It starts from scandium (Z = 21) and goes up to zinc (Z = 30).
The second transition series involves the filling of 4d-orbitals and includes 10 elements from yttrium (Z = 39) to cadmium (Z = 48).
The third transition series involves filling of 5d-orbitals. The first element of this series is lanthanum (Z = 57). It is followed by fourteen elements called lanthanoids which involve the filling of 4/-orbitals. The next nine elements from hafnium (Z = 72) to mercury (Z = 80) belong to third transition series.
The general electronic configuration of transition elements is (n -1) d1 – 10 ns1 – 2. The electronic configurations of the first row transition elements are given in Table 19.1.
Table 19.1. Electronic Configurations of Elements of the
First Transition Series (21Sc to 30Zn).
It may be noted that the electronic configurations of zinc, cadmium and mercury can be represented by the general formula (n- 1) d10 ns2. The d-subshell in these elements is completely filled in the ground state as well as in their common oxidation states. Because of this reason, the elements Zn, Cd and Hg are not regarded as transition elements.
Scandium (Sc) also does not exhibit typical properties of d-block elements because in Sc(III) which is the only oxidation state of scandium, it has no electron in the d-subshell.
Sc3+ : ls2, 2s2 2p6, 3s2 3p6