Electronic Configurations

All the actinoids are taken to share the outer configuration $7s^2$, with the remaining electrons distributed between the $5f$ and $6d$ subshells in a variable way. In principle, fourteen electrons are added progressively to the $5f$ subshell across the series — but not strictly in order. Thorium ($Z=90$) does not begin filling $5f$ at all; the $5f$ orbitals only start filling from protactinium onward, and they are not completely filled until lawrencium, element 103.

This irregular filling pattern mirrors what happens in the lanthanoids, and for the same underlying reason: extra stability is associated with an empty ($f^0$), half-filled ($f^7$), or completely filled ($f^{14}$) $5f$ subshell. This is why, for example, americium and curium have the configurations

— curium borrows an electron into $6d$ specifically so that its $5f$ subshell can stay at the extra-stable half-filled $5f^7$ configuration. …