Why is chlorine atom has atomic mass of 35.5 atomic mass unit

I’m not sure exactly what question you’re trying to ask, but I’ll give two answers:

1. The mass is reported as 35.5 amu because atomic weights are reported based on the weights and abundances of the isotopes of that element. For example, Hydrogen (which is typically understood to have only one proton, aka 1 amu) has a weight of 1.004 amu. This is because heavier isotopes of this element do exist but in very small quantities; however, they can still influence the “averaged” weight.

2. The weight of an atom is intrinsic based on its makeup - the number of each component influence the overall weight of the atom. In this case, Chlorine is comprised of the right number of protons and neutrons in the right occurrence and organization such that a weight of 35.5 amu is reported.

Hope this helps!