What does the term relative atomic mass, Aᵣ, refer to?

The term relative atomic mass, represented as Aᵣ, refers to the average mass of an atom of an element, measured relative to 1/12th of the mass of an atom of carbon-12. This definition is essential because it provides a standardized basis for comparing the masses of different atoms. Carbon-12 is chosen as a reference point because it is a stable isotope and has an exact mass of 12 atomic mass units (amu).

The relative atomic mass takes into account the natural isotopic distribution of the element, meaning it averages the masses of all the isotopes of that element based on their relative abundance. This allows scientists to understand and predict how elements behave in chemical reactions and calculations related to moles and stoichiometry.

In contrast, measuring the mass of a neutron compared to hydrogen or calculating the total number of protons and neutrons in the nucleus relates to atomic structure, not atomic mass in the relative sense. Additionally, the mass of a molecule compared to oxygen deals with molecular mass rather than atomic mass. Thus, defining Aᵣ in the context of carbon-12 provides clarity and essential context for atomic mass comparisons.