Atomic mass is described as?

• A. Mass of an atom usually expressed in atomic mass units.
• B. The number of protons times weight.
• C. The mass of nucleus only.
• D. The mass of electrons only.

Answer: (A)

Atomic mass is the total mass of an atom, usually expressed in atomic mass units. The unit, the dalton (amu), is defined so that one amu is 1/12 the mass of a carbon-12 atom, making it convenient to compare different atoms. This mass comes from all the subatomic particles: protons, neutrons, and electrons. Protons and neutrons add most of the mass, while electrons contribute much less, so the mass is dominated by the nucleus but still represents the whole atom. In practice, the atomic mass you see for an element is a weighted average of the masses of its naturally occurring isotopes, not a single nucleus. So the statement that atomic mass is the mass of an atom expressed in atomic mass units captures the full concept, whereas describing it as only the nucleus, only protons, or only electrons misses key parts of the picture.