Question

explain why atomic weight of chlorine is 35.5 the it has the mass number of 35

Answer 1

Here we go!!

Atomic weight is actually the wieghted average of the atomic masses of all the isotopes of chlorine.

Atomic weight = [% of first isotope of Chlorine x its atomic mass + % of second isotope x its atomic mass]/100

Thats why its a fraction and it wont be the same as the mass number of chlorine thats 35 which is actually the atomic mass of the stable isotope of chlorine.