nitrogen rule

Here is a small piece of mass-spectrometry magic that takes ten seconds to apply and instantly narrows the field. Just by reading whether the molecular weight is an even number or an odd number, you can often tell whether the molecule contains an odd number of nitrogen atoms. This shortcut is the nitrogen rule, and it works because nitrogen is the one common organic element with an odd valence (3) but an even atomic mass (14).

The rule states: a molecule made of C, H, O, N (and halogens) has an odd nominal molecular weight if and only if it contains an odd number of nitrogen atoms. Put plainly — no nitrogen, or two or four nitrogens, and the molecular weight comes out even; one or three nitrogens, and it comes out odd. The reason is a counting quirk: carbon, oxygen, and the halogens have even masses, hydrogen contributes one mass unit per atom, and only nitrogen's combination of even mass with odd valence shifts the total parity. So an odd M+ is a flag saying 'an odd number of nitrogens lives here.'

This is genuinely useful in practice. Faced with an unknown molecular ion at, say, m/z 93, the odd value immediately tells you the molecule likely contains one nitrogen — which, paired with an IR N-H stretch, might point to aniline (C6H5NH2, mass 93). Many drugs, alkaloids, and biological amines contain nitrogen, so this even/odd check is often the first thing a chemist notes when an unknown spectrum appears.