Chemistry – Stoichiometry - Relative masses of atoms and molecules | e-Consult

The atomic mass listed on the periodic table is not always a whole number because it represents the weighted average of the masses of all the naturally occurring isotopes of that element. As explained previously, isotopes have different masses, and their relative abundances are often not simple fractions. The atomic mass is calculated by taking the mass of each isotope, multiplying it by its relative abundance, and summing these values. This weighted average often results in a decimal value.

Example: Chlorine (Cl) has two major isotopes: ³⁵Cl, which occurs with a relative abundance of approximately 75.77%, and ³⁷Cl, which occurs with a relative abundance of approximately 24.23%. The atomic mass of chlorine is listed as approximately 35.45 amu. This is calculated as: (35 x 0.7577) + (37 x 0.2423) = 26.52 x 0.7577 + 8.97 x 0.2423 = 20.02 + 2.18 = 35.45 amu. The decimal value is a direct result of the weighted average of the two isotopes' masses and abundances.