Atomic Mass refers to which of the following?

• A. Sum of weight of protons and neutrons in an atom
• B. Sum of protons and neutrons
• C. Number of protons
• D. Number of electrons

Answer: (A)

Atomic mass is the total mass that makes up the atom, with the bulk coming from the nucleons in the nucleus. Protons and neutrons contribute nearly all of the atom’s mass, while electrons carry far less mass, so the overall atomic mass is well described by adding the masses of protons and neutrons. In practice, this is why the atomic mass is close to the mass number (the sum of protons and neutrons), though the exact value is slightly reduced by binding energy (the mass defect) and includes a tiny electron contribution. So the option describing the mass as the sum of the masses of protons and neutrons best captures what atomic mass represents.