Diatomic molecule: Contains two atoms covalently bonded together

The diagram below shows the mass spectrum of chlorine, which exists as diatomic molecules \(Cl_2\)

There are two peaks corresponding to the isotopic masses 35 and 37. These peaks have abundances approximately in a 3:1 ratio.

This allows us to calculate the relative atomic mass of chlorine:

\[ \text{Relative atomic mass = }\frac{ (3\times 35) + (1\times 37)}{4} = 35.5\]

This also allows us to calculate the abundance of each chlorine isotope.

Cl-35 has 75% abundance, while Cl-37 has 25% abundance i.e. The chance of selecting one Cl-35 atom is 0.75, while the chance of selecting one Cl-37 atom is 0.25

There are also peaks corresponding to the masses 70,72 and 74. These are due to the formation of diatomic molecules that combine different isotopes (i.e. both of the chlorine atoms in the molecule do not have the same mass)

These peaks have an approximate abundance ratio of 9:6:1.

The peak at 70 is due to the combination of two Cl-35 atoms. The chance of selecting one Cl-35 atom is 0.75, so the chance of selecting two is \(0.75 \times 0.75= 0.5625\)

The peak at 72 is due to the combination of one Cl-35 atom with one Cl-37 atom. The chance of this is \(0.75 \times 0.25= 0.375\)

The peak at 74 is due to the combination of two Cl-37 atoms. The chance of this is \(0.25 \times 0.25= 0.0625\)

The ratio of these abundancies , 0.5625:0.375:0.0625, is the same as 9:6:1.