Question

The atomic mass of chlorine 35.5 u. But when we add no. Of protons and neutron it atomic mass is 35u.

Answer 1

Heya,

There are two isotopes of chlorine 
1.Chlorine-37
2.Chlorine-35

Chlorine -37 in nature occurs about 25% and Chlorine-35 occurs about 75%.

So pure chlorine contains 75% of Chlorine-35 and 25% of Chlorine-37.

Mass of chlorine is measured and the result
obtained is 35.

It's known as atomic weight or relative mass of chlorine.

HOPE IT HELPS:-))