Nitrogen Rule Practice
Whether the molecular ion’s mass is even or odd is the fastest piece of information in any mass spectrum: an odd nominal mass means an odd number of nitrogen atoms, always, for ordinary organic compounds. Here you read a nominal M⁺• and pick the strongest claim it supports about the nitrogen count. The structure appears after you answer.
Common Questions
What is the nitrogen rule in mass spectrometry?
For a compound made of C, H, N, O, S, halogens, and P, the nominal mass of the molecular ion M⁺• is odd exactly when the molecule contains an odd number of nitrogen atoms. Zero or an even count of nitrogens gives an even nominal mass. It works in both directions: an odd molecular ion tells you to look for one (or three) nitrogens, and a formula with two nitrogens must sit at an even mass.
Why is nitrogen the only element that affects whether the mass is even or odd?
Every other common element pairs an even mass with an even valence (C is 12/4, O is 16/2, S is 32/2) or an odd mass with an odd valence (H is 1/1, Cl is 35/1, P is 31/3), so a molecule built from them alone always comes out with an even mass. Nitrogen is the only one that has an even mass (14) but an odd valence (3). Each nitrogen therefore switches the total mass between even and odd.
Does the nitrogen rule work for [M+H]⁺ ions in ESI mass spectrometry?
Not as first taught: it inverts. Protonation adds one hydrogen, so an even-nitrogen compound whose M⁺• would be even shows an odd [M+H]⁺. The rule itself still holds; you just have to apply it to the neutral molecule, not the protonated ion. Our Exact Mass page drills exactly this trap.