Why is atomic mass of chlorine taken as 35.5 u and not a whole number like 35 u or 36 u? Explain

cbse
structureofatom

#1

Chlorine in natural state comprises two isotopes of atomic masses 35 u and 37 u in the ratio 3:1. Hence, the atomic mass of chlorine is taken as the average of atomic masses of two isotopes. Thus,
Average atomic mass =
35 x 3/4 + 37 x 1/4 = 35.5 u