What is defined as the average mass of the isotopes of an element?

The average mass of the isotopes of an element refers to atomic weight, which takes into consideration the relative abundances of each isotope and their respective masses. Each isotope of an element has a different mass owing to variations in the number of neutrons in the atomic nucleus. The atomic weight is calculated by multiplying the mass of each isotope by its natural abundance (as a fraction of 1), and then summing these values. This value is often expressed in atomic mass units (amu) and provides a weighted average that reflects the isotopic composition of the element as it occurs in nature.

In contrast, molar mass refers to the mass of one mole of a substance and is numerically equivalent to the atomic weight but is expressed in grams per mole. Atomic number refers to the number of protons in the nucleus of an atom, which defines the element itself but does not give information about isotopes. Molecular weight pertains to the sum of the atomic weights of the atoms in a molecule and is relevant for compounds rather than individual elements.