What is the mass of an atom compared with a C-12 atom?

• A. Relative atomic mass
• B. Relative molecular mass
• C. Relative formula mass
• D. Mole

Answer: (A)

That ratio of an atom’s mass to 1/12 of a carbon-12 atom is the relative atomic mass. It’s a dimensionless number that tells you how heavy an atom is compared with the standard 1/12 of a carbon-12 atom. By definition, carbon-12 itself has an Ar of exactly 12, and other elements have Ar values that are the weighted averages of their isotopes.

This term is specific to atoms. Relative molecular mass would apply to whole molecules, and relative formula mass to formula units in compounds, not single atoms. A mole is a count of particles, not a mass ratio.