Transition metals : Elements having partially filled d-orbitals in ground state or in excited state, are known as transition elements.
The orbitals of the elements like Zn,Cd,Hg are completely filled when they are in their ground state as well as in their general oxidation state. Therefore, these elements are not considered as transition elements.
In transition elements, the variability in the oxidation state is due to the participation of (n−1)d orbitals and ns orbitals. Thus, oxidation states differ by unity. For example, Fe+3 to Fe+2,Cr+3 to Cr+2, etc On the other hand, the variable oxidation states shown by some p block elements differ by two units. For example, S4+ to Sn2+,Pb4+ to Pb2+.