Question

explain why chlorine always has relative atomic mass of about 35.5 u

Answer 1

because chlorine has two isotpes with masses 35u and 37u the average mass of both the isotopic chlorine is 35.5u

Answer 2

The mass of an atom of any natural element is taken as the averagass of all naturally occurring atoms of element. If an element occurs in isotope like chlorine,then we have to know the percentage of each isotope form and then the average mass is calculated.
So the average atomic mass of chlorine is 35.5
Hope it helps