Why is an element’s atomic weight usually not a whole number?

An element’s atomic weight is typically not a whole number because it represents the weighted average of the atomic masses of all naturally occurring isotopes of that element.3

While individual atoms have a specific mass number—the sum of their protons and neutrons—elements found in nature usually exist as a mixture of different isotopes with varying numbers of neutrons. Because these isotopes occur in different proportions, the atomic weight is calculated by averaging their individual masses based on their natural abundance.123

For example, while helium-3 and helium-4 have distinct masses, the atomic weight of helium is determined by the weighted contribution of each, resulting in a non-integer value. Additionally, the atomic mass unit itself is defined relative to carbon-12, and the actual mass of an atom is influenced by the mass of its electrons and the mass defect, further ensuring that atomic weights are rarely exact whole numbers.312