If a compound contains an even number of nitrogen atoms (or no nitrogen atoms), its molecular ion will appear at an even mass number. If, however, a compound contains an odd number of nitrogen atoms, then its molecular ion will appear at an odd mass value. This rule is very useful for determining the nitrogen content of an unknown compound. In the case of benzamide (Figure 1), the molecular ion appears at m/e = 121, indicating an odd number of nitrogen atoms in the structure.
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