Explain the exceptions in the general electronic configuration of $3d$-series elements.

(N/A) The exceptions in electronic configuration occur because of the very small energy difference between $(n-1)d$ and $ns$ orbitals.
$(i)$ The electronic configuration of $Cr$ is not $[Ar] 3d^4 4s^2$ but $[Ar] 3d^5 4s^1$.
$(ii)$ The electronic configuration of $Cu$ is not $[Ar] 3d^9 4s^2$ but $[Ar] 3d^{10} 4s^1$.
In $Cr$,the $3d^5$ configuration is formed because half-filled $d$-orbitals are more stable.
In $Cu$,the $3d^{10}$ configuration is formed because completely filled $d$-orbitals $(3d^{10})$ are more stable.
Due to the small energy gap between $3d$ and $4s$ orbitals,the half-filled $3d^5$ and fully-filled $3d^{10}$ configurations are relatively more stable.