Question

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

Hint: Chlorine has atomic no. 17 and it is present in the 17 groups of the halogens in period 3. There are two isotopes of the chlorine which have an atomic mass of 35 and 37 and they are found naturally on the earth with a percentage of 75% and 25% respectively.

$\text{35 }\times \text{ }\frac{3}{4}\text{ + }\text{37 }\times \text{ }\frac{1}{4}\text{ = 26}\text{.25 + 9}\text{.25 = 35}\text{.5u}$