Relative atomic mass is defined as the mass of one atom of an element relative to 1/12 of the mass of a carbon-12 atom.

• A. The average mass of the element's isotopes
• B. The mass of one atom of an element relative to 1/12 of the mass of a carbon-12 atom
• C. The mass of a mole of the element
• D. The mass of a sample of the element

Answer: (B)

The idea being tested is how atomic mass is measured on a standard scale. Relative atomic mass is a dimensionless quantity that tells you how heavy an atom is on average compared with 1/12 of the mass of a carbon-12 atom. Because elements exist as mixtures of isotopes with different masses, the average mass of those atoms is a weighted average that takes into account how abundant each isotope is. In practice, this average is expressed in atomic mass units, where one atomic mass unit is defined as 1/12 of the mass of carbon-12, so the relative atomic mass essentially equals the weighted average of the isotopic masses in amu.

Why this wording is best: it explicitly ties the average atomic mass to the carbon-12 standard, showing that the ratio is relative, not an absolute mass like a mole or a sample. The other ideas describe related concepts—such as the average mass of the isotopes without referencing the carbon-12 scale, or masses used for moles or samples—but they do not capture the relative, dimensionless comparison to the carbon-12 standard.