Covalency occurs when an element exchanges electrons with other atoms of the same or different elements in order to achieve a stable electrical state.
If an atom shares one electron, its covalency is one.
It has a covalency of two if it can share two electrons.
Covalency of Nitrogen in :
Nitrogen () has a total of five valence electrons. It requires three electrons to complete its octet. It may share three valence electrons with the nitrogen atom to generate by forming a stable electronic state.
In , Nitrogen has a proclivity for pi-pi multiple bonds.
Nitrogen shares four pairs of electrons with oxygen, hence nitrogen covalency in is four.
Thus, the covalency of nitrogen in is 4 because three are covalent bonds and one is a coordinate bond.