Question

The ionisation enthalpies of the elements of the third transition series are higher than
the first and second series. why?

Answer 1

Dear Student

First ionisation enthalpies of 5d elements are higher than those of the 3d and 4d element.
Concept : Poor shielding effect.
Reason :
Mn: [Ar] 3d⁵ 4s²
Tc: [Kr] 4d⁶ 5s¹
Re: [Xe] 4f¹⁴ 5d⁵ 6s²

According to above electronic configuration
Mn - 3d series
Tc - 4d series
Re - 5d series

The difference in the Mn, Tc and Re is that Re is an elements of 5d series which contain 4f¹⁴ electron whereas f orbital is not involved in the Mn and Tc. That is 5d series elements contain 14 f electron which show poor shielding effect due to which attraction between outer valence electron and nucleus increasers and it become difficult to remove first electron i.e more energy is required to remove electron as compare to 3d and 4d series elements. Hence First ionisation enthalpies of 5d elements are higher than those of the 3d and 4d element.

Regards