Explain why electronic configurations of lanthanoids are not known with certainty.

Explanation

In the lanthanoids, the outermost 6s-subshell remains filled with two electrons `(6s^(2))`. But due to very small difference in energy between 4f and 5d-orbitals, the incoming electron may enter either of these two orbitals. This is responsible for causing irregularities in the electronic configurations of lanthanoids. Further irregularities are also related to the extraordinary stabilities of `f^(0), f^(7)` and `f^(14)` electronic configurations. Hence electronic configurations of lanthanoids are not known with certainty. As an illustration, it is really difficult to predict whether the configuration of Ce (Z = 58) is `[Xe]4f^(2)5d^(0)6s^(2)` or `[Xe]4f^(1)5d^(1)6s^(2)`.