How many different masses of the Cl2 molecule would you expect to find in a sample of chlorine gas?

There are two isotopes of Cl, namely ³⁵Cl and ³⁷Cl. Their atomic masses are 35 and 37 respectively.

Consider the molecular masses of Cl₂ molecules in a sample of chlorine gas:
For ³⁵Cl−³⁵Cl molecules, molecular mass = 35 × 2 = 70
For ³⁵Cl−³⁷Cl or ³⁷Cl−³⁵Cl molecules, molecular mass = 35 + 37 = 72
For ³⁷Cl−³⁷Cl molecules, molecular mass = 37 + 37 = 74

Hence, there are 3 different masses for Cl₂ molecules.

Two naturally occurring isotopes:

Cl-35 + Cl-35

Cl-35 + Cl-37 (Cl-37 + Cl-35 is considered to be the same.)

Cl-37 + Cl-37

Three.