Why is Chlorine element having fractional atomic mass?

Answer 1

The Atomic Mass is based on the average of the atomic mass of each isotope, based on how common the isotopes are.

From Wikipedia: Chlorine has 9 isotopes with mass numbers ranging from 32 to 40. There are two principal stable isotopes, 35Cl (75.77%) and 37Cl (24.23%), found in the relative proportions of 3:1 respectively, giving chlorine atoms in bulk an apparent atomic weight of 35.5.