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 :
ionisation enthalpies

Explanation

Ionisation enthalpy : In each series, the first ionisation enthalpies increase with increasing atomic number, although there are exceptions. In the same vertical column, the first ionisation enthalpies of some elements of 4d - series are higher, while some of them have lower values than the elements of 3d - series. However, in the same vertical coloumn the first ionisation enthalpies of the elements of 5d - series (3rd transition series) are always higher than those of 3d - and 4d - series. This is because of poor shielding of nuclear charge by 4f - electrons in the third transition series.