What is atomic weight based on?

Atomic weight is determined by the average masses of an element's isotopes and their relative abundances. This concept reflects how the different isotopes of an element, which possess the same number of protons but varying numbers of neutrons, contribute to the overall mass of the element as found in nature.

For instance, if an element has multiple isotopes, some of which are more abundant than others, the atomic weight calculation takes into account the mass of each isotope weighted by how common it is. This is why atomic weight is not merely a reflection of the mass of a single isotope but rather a representation of a mixture of isotopes.

The incorrect options do not capture this nuanced understanding: focusing solely on protons neglects isotopes, considering total mass overlooks the importance of abundance, and using the mass of the heaviest isotope ignores the contributions of lighter, more abundant isotopes. Thus, the precise definition of atomic weight hinges on both the average mass of all isotopes and their relative abundances in nature.