What do you call the sum of the number of protons and neutrons in an atom?

The sum of the number of protons and neutrons in an atom is referred to as the mass number. This value is significant because it provides insight into the isotope of an element, with isotopes differing in the number of neutrons while the number of protons (which determines the element itself) remains constant. The mass number is essential for understanding atomic structure, as it influences the stability and behavior of the atom in reactions.

In contrast, the atomic number strictly counts the number of protons within the nucleus and defines the element's identity. Molecular mass refers to the total mass of a molecule, factoring in all the atoms present in its structure. Elemental weight is not a standard term used in atomic theory and does not have a specific definition in this context. Thus, the term mass number is the most accurate and relevant choice for describing the sum of protons and neutrons in an atom.