The atomic weight (or atomic mass) of an element is the average relative mass of the naturally occurring atoms of that element. Both the periodic table and the alphabetical list of the elements show the atomic weights of the elements. The atomic weight of an element is based on the variety of naturally occurring isotopes of that element and the relative abundance of each.

A collection of naturally occurring carbon atoms contains 98.89% carbon-12 atoms and 1.11% carbon-13 atoms, along with a trace percentage of carbon-14 atoms. The atomic weight of carbon (12.01) reflects the relative abundance of these three isotopes. The atomic weight of chlorine (35.45) reflects the fact that 75.53% of naturally occurring chlorine is chlorine-35 and 24.47% is chlorine-37.

The atomic weight of some elements is given as a whole number enclosed in parentheses. These elements are unstable; that is, their nuclei decompose radioactively. The number in parentheses is the mass number of the most stable or best-known isotope of that element.

Atomic weights are measured in atomic mass units. One atomic mass unit is defined as 1/12 the mass of an atom of carbon-12. With this reference standard, no element has an atomic weight less than one. The approximate atomic weight of an element can be calculated if the relative abundance of its isotopes is known.

If the identity of the naturally occurring isotopes and the atomic weight of an element are known, it is possible to estimate which of the isotopes is most abundant.