What is relative atomic mass?

• A. The weighted mean mass of an atom relative to 1/12th of the mass of a carbon-12 atom
• B. The mass of a carbon-12 atom
• C. The mass of a hydrogen atom
• D. The atomic number of the element

Answer: (A)

Relative atomic mass is a way to compare how heavy an atom is on a standard scale. It’s the weighted mean mass of an atom relative to 1/12 of the mass of a carbon-12 atom. Because elements have different isotopes with slightly different masses, Ar is the average value you’d expect when you account for how common each isotope is. The result is dimensionless and is usually expressed on the carbon-12 scale, with carbon-12 defined as exactly 12. So Ar isn’t the mass of one specific isotope or of carbon-12 itself; it’s the average mass of all the atom’s isotopes weighted by their abundances.