Write down about the Hund's rule.

(N/A) Hund's rule of maximum multiplicity deals with the filling of electrons into orbitals belonging to the same subshell.
Rule: Pairing of electrons in the orbitals belonging to the same subshell ($p, d,$ or $f$) does not take place until each orbital of that subshell is singly occupied.
This means that electrons will first occupy all available orbitals in a subshell singly with parallel spins before pairing begins.
For example,in a $p$-subshell,the $4^{th}$ electron starts the pairing process. Similarly,for $d$ and $f$ subshells,pairing starts with the $6^{th}$ and $8^{th}$ electrons,respectively.
Example: Nitrogen $(Z=7)$ has the configuration $1s^2 2s^2 2p_x^1 2p_y^1 2p_z^1$,and Oxygen $(Z=8)$ has the configuration $1s^2 2s^2 2p_x^2 2p_y^1 2p_z^1$.