The mass of Cl on the periodic table is 35.45 atomic mass units. This is an average mass of the isotopes (different Cl atoms in nature). Most Cl atoms in nature have a relative mass of about 35 atomic mass units. Most of the remaining Cl isotopes have a relative mass of 37 atomic mass units.
There are about three Cl-35 isotopes for each Cl-37 isotope, so if you add up
35 + 35 + 35 + 37, and divide the total by 4, you come out with an average mass of about 35.5 atomic mass units.
There are other Cl isotopes but they make up a much lower percentage than the Cl-35 and Cl-37 varieties.
