Question

Chlorine has atomic mass 35.5, why

Answer 1

Everyone has given the right answers, but I'll try to make it more clear:

Chlorine-35 has relative atomic mass 35

Chlorine-37 has relative atomic mass 37

But the chlorine commonly found has both of these isotopes in the ratio of 3:1 (by mass) respectively

So we take weighted average and say that chlorine has relative atomic mass 35.5, because that's what will give you the most accurate results in calculations where you require the atomic mass of chlorine

Hope this helps ;)

Answer 2

Chlorine has atomic mass 35.5 because it has something called isotopes