Nitrogen Rule Practice
The parity of the molecular ion 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 mass gives you the parity, and nothing more. 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 flips the mass parity?
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 the parities cancel when you assemble a molecule. Nitrogen is the mismatch: even mass (14) but odd valence (3). Each nitrogen therefore flips the parity of the total mass, and only the nitrogen count decides odd versus even.
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.