Compare the general characteristics of the first series of the transition metals with those of the second and third series metals in the respective vertical columns. Give special emphasis on the following points:
(i) electronic configurations
(ii) oxidation states
(iii) ionisation enthalpies and
(iv) atomic sizes.

Explanation

(i) Electronic configurations : In the first transition series, 3d orbitals are progressively filled whereas in second and third transition series 4d and 5d orbitals respectively are filled.
(ii) Oxidation states: Elements show variable oxidation states in all the transition series. For the first transition series, the lower oxidation states especially+2 and +3 are more stable than for the elements of second and third transition series. For example, in cobalt group, cobalt forms a large number of complexes in Co (II) and Co (III) states, but no similar complexes of `Rh^(2+) or Rh^(3+) and Ir^(2+) or Ir^(3+)` are known. Within a group, the maximum oxidation state increases as we move from element of first transition series to second and third transition series. For example, in group 8, iron shows common oxidation states of +2 and +3 but ruthenium and osmium in the same group form compounds in +4, +6 and +8 oxidation states.
(iii) Ionisation enthalpies : The ionisation enthalpies of elements of 5d series are higher than those of 3d and 4d series. This is because in 5d series, there are filled 4f-orbitals. The 4f orbitals have poor shielding effect. As a result, the outer electrons have greater effective nuclear charge acting on the outermost electrons. Therefore, the ionisation enthalpies are higher.
(iv) Atomic size : The atomic size of 4d series are larger than those of 3d-series. However, the elements of 5d series have nearly the same size as those of 4d series because of lanthanoid contraction.