Mass spectrometry

The masses of atoms and molecules can be determined using a mass spectrometer. The masses can be shown as a series of peaks.

Element X exists as a diatomic molecule. The mass spectrum will show the mass of ions from individual atoms of X and from X2 molecules.

The mass spectrum for element X is shown below:

How many different isotopes of X can be determined from this spectrum?

A 1
B 2
C 3
D 4
E 5

The answer is B 2, but I don’t really get it. Any help would be appreciated!


atoms x can have 2 isotopes: ³⁵Cl or ³⁷Cl
as a diatomic molecule they can have a relative mass of
70 = ³⁵Cl + ³⁵Cl
72 = ³⁵Cl + ³⁷Cl
74 = ³⁷Cl + ³⁷Cl

Thus only 2 isotopes are identified in this spectrum

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