Relative atomic mass is measured on a scale where the carbon-12 atom has a mass of 12.

• A. The mass of one atom relative to 1/12 of a carbon-12 atom
• B. The mass of a molecule relative to hydrogen
• C. The mass per mole
• D. The mass of the entire sample

Answer: (A)

Relative atomic mass is a way to express how heavy an atom is compared with 1/12 of a carbon-12 atom. Because carbon-12 is defined to have a mass of exactly 12, the mass of any atom on this scale is given as a number that tells you how many times bigger or smaller it is than 1/12 of a carbon-12 atom. This makes the relative atomic mass a dimensionless quantity (often tied to the atomic mass unit), reflecting the atom’s mass in units of 1/12 of carbon-12. For example, an atom with a relative atomic mass of about 16 is about 16 times as heavy as 1/12 of carbon-12. The other descriptions refer to different concepts: mass per mole is molar mass (g/mol), the mass of a molecule relative to hydrogen describes relative molecular mass, and the mass of an entire sample is just total mass, not the atomic comparison.