The atomic mass listed on the periodic table is actually a weighted average of all of an element's naturally occurring isotopes, based on how abundant each one is.
The atomic mass listed on the periodic table is actually a weighted average of all of an element's naturally occurring isotopes, based on how abundant each one is. This calculator finds that weighted average from two isotopes' masses and their natural abundances.
Chlorine has two isotopes: Cl-35 (mass 34.97 amu, 75.77%) and Cl-37 (mass 36.97 amu, 24.23%). Average = (34.97×0.7577) + (36.97×0.2423) ≈ 35.45 amu — matching chlorine's known atomic mass.
It's the weighted average of an element's isotopes: sum of (isotope mass × its natural abundance fraction) for each isotope.
Because it blends the masses of multiple isotopes (which have slightly different masses due to differing neutron counts) weighted by how common each one is in nature.
They're measured experimentally and published by bodies like IUPAC; this calculator lets you enter your own values or use commonly cited ones.