Work out average atomic mass instantly with clear inputs, formula shown and shareable results.
The atomic mass on the periodic table is an abundance-weighted mean of the isotopes, not any single nuclide's mass. Chlorine is 75.76% Cl-35 (34.96885 u) and 24.24% Cl-37 (36.96590 u), which averages to 35.45 u. Because the weighting is by number of atoms, the mean always sits nearer the more abundant isotope.
Weighted mean
average = (m1 x a1 + m2 x a2) / 100
Abundance closure
a2 = 100 - a1
Because it is a mixture. No chlorine atom has a mass of 35.45 u - three quarters are 35 and one quarter is 37.
The same weighted mean extends to any number of terms. This two-isotope form covers the common exam cases such as chlorine, boron and copper.