Question

Why is the electron configuration of Lanthanum [Xe] 5d¹ 6s² instead of [Xe] 6s² 4f¹?

Answer 1

Lanthanum (La) is part of the d-block even though they are separated from the rest of the d-block by the f-block.

As the number of energy levels and sublevels increases it becomes complex enough so that the energy levels overlap. In terms of the electrons they are most stable in the ground state or lowest possible energy. This means that we fill the lowest energy levels first.

In Lanthanum, the empty 5d sublevel has lower energy than the 4f sublevel

By the addition of one electron to the 5d raises the energy of the level so that the next electrons go to 4f. Hence Lanthanum [Xe] 5d1 6s2 instead of [Xe] 6s2 4f1.