State the general electronic configuration of lanthanoids. Why are there irregularities in the electronic configuration of lanthanoids?

(N/A) The general electronic configuration of lanthanoids is $[Xe] 4f^{1-14} 5d^{0-1} 6s^{2}$.
Reason for irregularities:
The irregularities in electronic configurations arise because the energy difference between the $4f$ and $5d$ orbitals is very small.
Consequently,the incoming electron can enter either the $4f$ or the $5d$ orbital.
In lanthanoids,$f$-orbitals are generally filled successively with an increase in atomic number.
However,in elements like Lanthanum $(La)$,Gadolinium $(Gd)$,and Lutetium $(Lu)$,one electron is present in the $5d$ orbital.
These irregularities are due to the extra stability associated with empty $(f^{0})$,half-filled $(f^{7})$,and fully-filled $(f^{14})$ $f$-orbital configurations,respectively.
As Lanthanum and Lutetium possess one $5d$ electron,they are sometimes classified as group $3$ elements.