What is Lanthanoid contraction ?
Explain , why lanthanum `(Z=57)` forms `La^(3+)` ion, while cerium `(Z=58)` forms `Ce^(4+)` ion ?

Explanation

La has the electronic configuration of [Xe] `4f^(0) 5f^(1), 6s^(2)` after losing 3 electrons La forms `La^(3+)` ion which is stable due to empty 4f orbitals while Ce has the electronic configuratin of [Xe] `4f^(2),5d^(0) 6s^(2)` after losing 4 electrons Ce forms `Ce^(4+)` ion which is stable due to empty 4f orbitals. Thus, Lanthanum forms `La^(3+)` ion while cerium forms `Ce^(4+)` ions.