If the nitrogen atom has electronic configuration $1s^7,$ it would have energy lower than that of the normal ground state configuration $1{\mathrm{s}}^2,2{\mathrm{s}}^{2}2{\mathrm{p}}^3,$ because the electrons would be closer to the nucleus. Yet $1s^7,$ is not observed because it violates.

As per Pauli Exclusion Principle “no two electrons in the same atom can have all the four quantum numbers equal or an orbital cannot contain more than two electrons and it can accommodate two electrons only when their direction of spins are opposite”.