If the molecular mass of an unknown compound to the nearest integer value is an odd number, the compound contains an odd number of nitrogens in its molecular formula. Correspondingly, if the molecular mass is an even number, the compound contains zero or an even number of nitrogens in its molecular formula. This rule, illustrated below, results from nitrogen having a valence of three and an even atomic mass. Consistent with the nitrogen rule, a correct molecular formula for a molecule with an odd molecular mass in the nearest integer value will have the sum of the number of hydrogens plus halides as an odd number. Correspondingly, a correct molecular formula for a molecule with an even molecular mass will have the sum of the number of hydrogens plus halides as an even number.

Examples

The molecular ion for aminoethane (ethylamine), [CH₃CH₂NH₂]⁺, is m/z=45 amu, an odd number; the number of hydrogens is five, also an odd number.

The molecular ion for 1,2-diaminoethane, [NH₂CH₂CH₂NH₂]⁺, is m/z= 60 amu, an even number; the number of hydrogens is eight, also an even number.

Next section: Fragment Ions

Copyright information: Original content © University of Colorado, Boulder, Chemistry and Biochemistry Department, 2011. The information on these pages is available for academic use without restriction.

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