Atomic Mass exists as a whole number, why do we write the atomic mass of chlorine as 35.5.

 Chlorine has two isotopes and the mass of an atom is taken as the average mass of all the naturally occurring atoms of that element.

This is obtained by knowing the percentage of each isotopic form and then the average mass is calculated CL=35-75% and CL + 37-25%

= (35x75/100) + (37x25/100)

= 105/4 + 37/4

= 142/4

= 35.5 u