Nitrogen has many oxides, with varying degrees of electron sharing. These oxides range from colorless gases ( and NO), to a brown gas (), to an explosive white solid ().

If these all were ionic compounds rather than covalent molecules, and if each oxygen atom took two electrons from a nitrogen atom to complete its neon shell, then the formulas of the oxides given above would suggest that nitrogen gave up one electron in (recall ), two electrons in NO, four in , and all five in .

This is not what really happens; electrons in these molecules are shared rather than given up. But this formal accounting scheme leads to a useful quantity, the oxidation number of nitrogen, which ranges from +1 through +5 in these oxides.

The oxidation number is the charge that the nitrogen atom would have if both electrons in each covalent bond were given to the more electronegative oxygen atom.

We will return to the important concepts of oxidation and oxidation numbers in Chapter 6.