Relative Atomic Mass
Naturally occurring chlorine consists of 35Cl (34.9689 amu) and 37Cl (36.9659 amu). Which isotope is most abundant?
The relative atomic mass of chlorine is 35.45 amu, as shown on the periodic table. Since the relative atomic mass is closer to the mass of 35Cl (34.9689 amu) than the mass of 37Cl (36.9659 amu), the 35Cl isotope must be most abundant. (The actual percentages are 75.77% 35Cl and 24.23% 37Cl.)