The electronic configurations of actinides are not known with certainty. Explain.

Explanation

In the actinoids, the outermost 7s subshell remains filled with two electrons `(7s^(2))`. But due to very small difference in energy between 5f and 6d orbitals, the incoming electron may occupy either 5f or 6d subshell. This is responsible for causing irregulaties in the electronic configurations of actinoids. Further irregularities are also related to the extraordinary stabilities of `f^(0), f^(7)` or `f^(14)` electronic configurations. Hence electronic configurations of actinoids are not known with certainty. Thus for Ac (Z = 89), the expected configuration is `[Rn]5f^(1)6d^(0)7s^(2)` but it is actually `[Rn]5f^(0)6d^(1)7s^(2)`.