Co-ordinate (dative covalent) bonding
A covalent bond is formed by two atoms sharing a pair of electrons. The atoms are held together because the electron pair is attracted by both of the nuclei.
In the formation of a simple covalent bond, each atom supplies one electron to the bond - but that doesn't have to be the case. A co-ordinate bond (also called a dative covalent bond) is a covalent bond (a shared pair of electrons) in which both electrons come from the same atom.
For the rest of this page, we shall use the term co-ordinate bond - but if you prefer to call it a dative covalent bond, that's not a problem!
The nitrogen centre is formally quaternized.Explanation:
For H3CNO2, we have 3×1(H)+4(C)+5(N)+2×6(O)=24 valence electrons to distribute over 7 centres.
A Lewis representation of H3C−+N(=O)(−O−) is standard, and this distributes the 12 electron pairs. Because there are 6 electrons formally associated with nitrogen (4 from the bonds, and 2 inner core), this centre is said to be quaternized, and thus bears a formal positive charge (7 electrons are associated with the neutral nitrogen atom). The anionic oxygen is associated with 9 electrons (2 inner core) instead of the eight of neutral oxygen, and so bears a formal negative charge. Of course, we can distribute this negative charge to the other oxygen by resonance.