Relative atomic mass is measured relative to which standard isotope?

• A. Carbon-12
• B. Hydrogen-1
• C. Oxygen-16
• D. Nitrogen-14

Answer: (A)

Carbon-12 is used as the reference because its mass is defined to be exactly 12 atomic mass units. This sets the scale for atomic masses: one atomic mass unit is defined as 1/12 of carbon-12’s mass. Relative atomic mass is the weighted average mass of an element’s isotopes measured against this standard, so it tells you how heavy an atom is compared to carbon-12. The other isotopes listed exist with different masses, but they do not define the scale itself; hydrogen-1 is close to 1 u, while oxygen-16 and nitrogen-14 are just common isotopes used in calculations, not the fixed reference for the unit.