What does relative atomic mass compare?

Relative atomic mass is a measure used to express the mass of an atom compared to a standard reference. The standard reference for relative atomic mass is one-twelfth of the mass of a carbon-12 atom. This definition is crucial because it provides a consistent and precise way to compare the masses of different elements. By using carbon-12, which is a stable isotope, chemists have a reliable benchmark to work from, allowing for accurate calculations and comparisons between different elements.

This definition ensures that the relative atomic mass of an element takes into account the isotopes present in nature and their abundances, which is essential for understanding the overall behavior of the element in chemical reactions. Thus, the correct answer highlights that relative atomic mass is specifically a ratio relative to the mass of a carbon-12 atom rather than other factors such as hydrogen or isotopic averages, making it a foundational concept in chemistry for measuring atomic mass.