Hund's Rule of Maximum Multiplicity states that for degenerate orbitals (orbitals with same energy like $p_x,p_y,p_z$), pairing of electrons does not occur until each orbital is singly occupied.

• Nitrogen ($Z=7$) as per Aufbau: $1s^{2}2s^{2}2p^3$
• For the $2p$ subshell, the 3 electrons are placed in $2p_x,2p_y$, and $2p_z$ singly instead of pairing up.
• This configuration $(\uparrow ,\uparrow ,\uparrow )$ minimizes electron-electron repulsion and maximizes exchange energy stability.