Question

why actenium and thorium have irregulur electronic configuration

Answer 1

The electronic configuration of thorium (Z=90) is 5f06d27s2. But, according to the aufbau principle, the electrons should first enter the f subshell and not the d subshell. This is not a special case of a half-filled subshell or a fully-filled subshell like we see in the respective cases of chromium and copper. Then why do we write the electronic configuration like this, first filling the d subshell and not the f

Answer 2

Answer:

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.