What does atomic mass describe?

• A. Atomic mass
• B. Atomic number
• C. Isotopes
• D. Binding energy

Answer: (A)

Atomic mass describes how heavy an atom is. It is the total mass of the atom, typically measured in atomic mass units, and is dominated by the mass of the nucleus—the protons and neutrons it contains. Electrons contribute only a tiny amount, so the atomic mass is a close reflection of the sum of protons and neutrons, i.e., the nucleons. However, the actual atomic mass isn’t just a simple integer; it’s slightly less than the sum of the individual masses due to binding energy that holds the nucleus together (mass defect). For elements with multiple isotopes, the atomic mass published is a weighted average of the masses of those isotopes based on their natural abundances.