The mass of Cl on the periodic table is 35.45 atomic mass units. This is an average mass of the isotopes of chlorine. 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.
The number 35.45u is the result of the natural mixture of Cl-35 and Cl-37, which produces the observed value.
