Anomalous Electronic Configuration of Thorium

Question

The electronic configuration of thorium ($Z=90$) is $5\mathrm f^0 6\mathrm d^2 7\mathrm s^2$. But, according to the aufbau principle, the electrons should first enter the $\mathrm f$ subshell and not the $\mathrm d$ 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 $\mathrm d$ subshell and not the $\mathrm f$ subshell?

P.s I came across the anomalous configuration of thorium here: https://en.wikipedia.org/wiki/Thorium#cite_ref-Wickleder5960_19-0

But it doesn't state the reason for the anomaly.

Answer 1

For first four actinide elements $\ce{Th}$, $\ce{Pa}$, $\ce{U}$ and $\ce{Np}$, the difference in energy between $\mathrm{5f}$ and $\mathrm{6d}$ orbitals is small. Thus in elements electrons may occupy the $\mathrm{5f}$ or $\mathrm{6d}$ levels or sometimes both.

Later in actinide series the $\mathrm{5f}$ do become appreciable lower in energy, thus from $\ce{Pa}$ onward the $\mathrm{5f}$ shells fill in regular way.