# Exceptions to the Nitrogen Rule in mass spectrometry

The Nitrogen Rule in mass spectrometry states that (organic) molecules containing no or an even number of nitrogen atoms will have even masses, and molecules containing an odd number of nitrogen atoms will have odd masses.

Examples (all monoisotopic masses):
No nitrogen, even mass:
H2, 2 Da
HBr, 80 Da
C2H6, 30 Da
One nitrogen, odd mass:
NH3, 17 Da
C5H5N, 79 Da
etc.

Often there is a little disclaimer saying that the nitrogen rule is only valid for molecules that do not contain "exotic atoms." What are some examples for molecules do not follow the nitrogen rule?

• Just look at periodic table. P and Cl have odd masses. – MaxW Nov 5 '18 at 23:30
• @MaxW - Yes, but P tends to be tri or pentavalent, so that cancels out. Consider phosphoramide $\ce{H6N3OP}$, mass 95 Likewise with Cl - its monovalent. Chloramine $\ce{H2NCl}$, mass 51 (and 53). What you need is an even valent atom with an odd mass or an odd valent atom with an even mass (like N). – Ben Norris Nov 6 '18 at 3:26

EDIT: I spoke too soon about abundances of nuclides with an odd number of neutrons. Here is a table of such nuclides with abundance $$> 20\%$$ taken from http://atom.kaeri.re.kr/nuchart/ : $$\begin{array}{rl} \text{Nuclide} & \text{Abundance} \\ \hline \sideset{^{9}}\ {Be}& 100\%\\ \sideset{^{14}}\ {N}& 99.636\%\\ \sideset{^{105}}\ {Pd}& 22.33\%\\ \sideset{^{129}}\ {Xe}& 26.4006\%\\ \sideset{^{131}}\ {Xe}& 21.232\%\\ \sideset{^{163}}\ {Dy}& 24.896\%\\ \sideset{^{167}}\ {Er}& 22.869\%\\ \sideset{^{195}}\ {Pt}& 33.78\%\\ \sideset{^{207}}\ {Pb}& 22.1\% \end{array}$$ Of these $$\sideset{^{195}}\ {Pt}$$ is the most abundant isotope of $$Pt$$ in addition to $$\sideset{^{9}}\ {Be}$$ and $$\sideset{^{14}}\ N$$ previously mentioned.