What is the term for the mass of an atom, including its protons, neutrons, and electrons?

The term that refers to the mass of an atom, which includes the sum of its protons and neutrons along with its electrons, is called atomic mass. The atomic mass is typically expressed in atomic mass units (amu) and reflects the total mass of an individual atom, with protons and neutrons contributing most significantly to the mass since electrons have a much smaller mass.

Atomic weight, on the other hand, is an average of the atomic masses of an element’s isotopes, weighted by their natural abundance, which is distinct from the individual atomic mass of a single atom. Molecular mass pertains to the mass of a molecule, calculated by adding up the atomic masses of all the atoms in the molecule; this is not applicable when referring to a single atom. The term mass index is not a standard term used in chemistry or physics to describe atomic mass or weight. Thus, atomic mass is the most accurate and appropriate choice for this question.