Chlorine atoms have 2 isotopes: Cl−35 and Cl−37.Which isotopes is most abundant?

The average atomic mass of chlorine is 35.5 amu. 

Chlorine consists of two isotopes :$C{l}^{35}$and $C{l}^{37}$ .

It's average mass depends on the relative abundance of the two isotopes.

Let x be the relative abundance of $C{l}^{35}$ then (1-x) is the relative abundance of $C{l}^{37}$ .

Average atomic mass =X(35)+(1-X)37/X+(1-X)

35.5=35x+37−37x

1.5=2x

x=0.75

This implies that out of every four atoms of chlorine, there are 3 atoms of Cl-35 and 1 atom of Cl-37. Hence, Cl-35 is more abundant than Cl-37.