The electronic configuration of thorium (Z=90) is
. But according to aufbau principle, the electrons should first enter f subshell and not d subshell. This is also not a case of half filled subshell or full filled subshell like we see in the case of chromium and copper. Then why do we write the electronic configuration like this, first fill the d subshell not f subshell?
In actinium the energies of 5f, 6d and 7s are very close to one another and one electron enters the 6d orbital before 5f begins to fill. The outer electronic configuration of actinium is 6d1 7s2. In the subsequent elements the 5f sub-shell is stabilized relative to the 6d sub-shell.