Question

Explain why so many f block elements have ‘unusual’ experimental electron configurations. (This is for basic level chemistry so a simple answer is fine.)

Answer 1

In lanthanoid series, After the element Cerium (Ce), Gadolinium (Gd) and Lutetium (Lu) shows unusual configuration (not in line with other lanthanoids).

This is because, a certain electronic configuration is more stable when the sublevels (atomic orbital) are half filled or completely filled than when it is partiallly filled.

for example, The observed outer electronic configuration of Europium ( ⁶³Eu) is 4f⁷ 6s² and that of element Gadolinium ( ⁶⁴Gd) is 4f⁷ 5d¹ 6s² but Not  4f⁸ 6s² , because a half filled f-orbital (f⁷) is more stable than partially filled (f⁸ ) therefore the next electron is filled in 5d orbital.

A general configuration for actinides is (n−2)f^1-14 (n−1)d^0-1 ns² i.e. all actinides have 7s² outer electrons but variable.occupancy of 6d and 5f orbitals. From Protactium ( ⁹¹Pa) the 5f orbital is regularly filled (succesive electron got filled into 5f orbital) But Curium and Lawrencium shows unusual configuration than other actinides. Curium ( ⁹⁶Cm) is 5f⁷ 6d¹ 7s² rather than 5f⁸ 7s² because half filled (5f⁷) is more stable than partially filled (5f⁸) and therefore, the next electron goes to 6d orbital.

Also, as we go to higher energy levels (n) , the difference in energies of the (n-1)d-orbitals and (n-2)f-orbitals are quite less therefore, electron starts to fill in the (n-2)f orbitats instead of (n-1)d orbital.