Nitrate ion, NO₃^-

In the diagram, there is a double bond between nitrogen and the oxygen atom on the left. The extra electron is located on the top right oxygen atom which forms a single bond with N. The bottom right oxygen forms a single bond with N in which both shared electrons originate from N. This is a dative bond. In reality, all three oxygen atoms share the charge and are bonded identically to the N. Nitrate and carbonate are isoelectronic - they have the same number of electrons, and the only difference is that N has one more proton than C.

Dot-cross diagrams of ions