Atomic number of nitrogen is 7. Thus the electronic configuration of nitrogen is 1s2 2s2 2p3.
The valence shell, is the outermost shell of an element. Nitrogen has 5 electrons in its valence shell (n = 2).
Now, when gains 3 electrons to fill its outermost shell, it complete its octet. And when any system completes its octet, it becomes stable.