If the nitrogen atom had electronic configuration $1 \mathrm{~s}^{7}$ it would have energy lower than that…
If the nitrogen atom had electronic configuration $1 \mathrm{~s}^{7}$ 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 $1 \mathrm{~s}^{7}$ is not observed. It violates
Heisenberg's uncertainty principle
Hund's rule
Pauli exclusion principle
Bohr postulate of stationary orbits
Solution
Not more than two electrons can be present in same atomic orbital. This is Pauli's exclusion principle.
,