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 configuration . In the first transition series, 3d orbitals are progressively filled while in the seond and third transition series, 4d and 5d orbitals . Further , the elements in the same vertial 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^(7) 5s^(1) , Pd ( 46) = 4d^(10) 5s^(0), Ag( 47) = 4d^(10) 5s^(1)` . Similarly , in the third series, `W( 74) = 5d^(4) 6s^(2) , Pt ( 78) = 5d^(9) 6 s^(1) ` and `Au ( 79 ) = 5d^(10) 6 s^(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 is maximum and minimum at the extreme ends.
However , the first row elements differ from the second and thrid row elements in the fact that for all the first row elements ,`+2` and `+3` states are important , i.e., ions `M^(2+)` and `M^(3+)` are common but these are less important for second and thrid row elements. For example, first low elements form a large number of extremely stable complexes in `+ II` or `+ III` states , e.g., `[overset(III) (Cr)Cl_(6)]^(3-) ` and `[overset(III) (Co)(NH_(3))_(6)]^(3+)`. No equivalent complexes of Mo or W or Rh or Ir, are known . For second and third row elements , higher oxidation states are more important than those of the first row elements. For example, some compounds of second and third row elements exist in higher oxidation states which have no counter parts in the first row , e.g., `WCl_(6) , ReF_(7) , RuO_(4) , OsO_(4) ` and `PtF_(6)`
(iii) Ionization enthalpies. The first ionization enthalpies in each series generally increase gradually as we move from left to right though some exceptions are observed in each seres. The first ionization 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 ionization enthalpies of third ( 5d) series are hgiher 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. In general , ions ofthe same charge or atoms in a given series show progressively decrease in radius with increasing with atomic number through the decrease in quite small. But the size of the atoms of the 4d seris is larger than the corresponding element of the 3d series whereas those of correspongind elements of the 5d series are nearly as those of 4d series because of lanthanoid contraction.