Compare the general characteristics of the first series of 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 while in the second and third transition series, 4d and 5d orbitals are progressively filled. Elements in the same vertical column generally have similar electronic configurations. However, the first series shows only two exceptions, i.e., `Cr=3d^(5)4s^(1)` and `Cu=3d^(10)4s^(1)` but second series shows more exceptions, e.g., Mo `(42)=4d^(5)5s^(1),Tc(43)=4d^(6)5s^(1),Ru(43)=4d^(7)5s^(1),Rh(44)=4d^(8)5s^(1),Pd(46)=4d^(10)5s^(0),Ag(47)=4d^(10)5s^(1)`. Similarly, in the third series the exceptions are : W `(74)=5d^(4)6s^(2),Pt(78)=5d^(9),6s^(1)`. Thus, in the same vertical column, in a number of cases, the electronic configuration of the three series are not similar. (ii) Oxidation states : The number of oxidation states shown by the elements in the middle of each series in maximum and at the extreme ends, it is the minimum.
However, it has been observed that for all the first row elements, +2 and +3 states i.e, ions `M^(2+)` and `M^(3+)` are more common. For example, first row elements frorm a large number of extremely stable complexes in +II or +III states, e.g., `overset(III)([CrCl_(6)]^(3-))` and `overset(III)([Co(NH_(3))_(6)]^(3+))`. No equivalent complexes of Mo, W, Rh, Ir, are known. In second and third row elements, higher oxidation states are more common. For example, some compounds of second and third row elements exist in higher oxidation states which have no counterparts in the first row, e.g., `WCl_(6),ReF_(7),RuO_(4),OsO_(4)` and `PtF_(6)`.
(iii) Ionisation enthalpies : The first ionisation enthalpies in each series generally increase gradually as we move from left to right though some exceptions are observed in each series. The first ionisation enthalpies of some elements in the second (4d)series are higher while some of them have lower value than the elements of 3d series in the same vertical column. However, the first ionisation enthalpies of third (5d) series are higher than those of 3d and 4d series. This is because of weak shielding of nucleus by 4f electrons in the 5d series.
(iv) Atomic sizes : It is observed that, ions of the same clarge or aloms in a given series show progressive decrease in radius with increasing atomic number though the decrease is quite small. But the size of the atoms of the 4d series is larger than the corresponding elements of the 3d series whereas those of corresponding elements of the 5d series are nearly the same as those of 4d series because of lanthanoid contraction.