What is atomic weight defined as?

Atomic weight, also known as atomic mass, is defined as the weighted average mass of an element's isotopes compared to a baseline, typically expressed in atomic mass units (amu). It specifically reflects the sum of protons and neutrons in an atom's nucleus. Since the nucleus contains the majority of an atom's mass, the combined total of protons, which define the element, and neutrons, which contribute to the atom's mass but not its charge, provides a comprehensive understanding of how heavy the atom is.

This is especially important as different isotopes of an element can have varying numbers of neutrons, affecting the atomic weight. For example, the atomic weight of carbon is roughly 12 amu because it is primarily composed of isotopes with six protons and six neutrons. In contrast, the other options relate to different aspects of atomic structure rather than the concept of atomic weight. The number of protons defines the element but does not account for mass accurately. The number of electrons influences charge but not the atomic weight, and the mass of an atom, while related, does not clarify how atomic weight is calculated or expressed. Hence, understanding atomic weight fundamentally relies on the total count of protons and neutrons.