In the definition of relative atomic mass, carbon-12 refers to a specific isotope of carbon. Isotopes are variants of an element that have the same number of protons but differ in the number of neutrons in their nuclei. Carbon-12 specifically refers to the isotope of carbon that has 6 protons and 6 neutrons, giving it a mass number of 12.
In the context of relative atomic mass, carbon-12 is used as the reference isotope against which the atomic masses of other elements are compared. The relative atomic mass scale is based on the assumption that one atom of carbon-12 has a mass of exactly 12 atomic mass units (AMU). This choice provides a convenient and consistent standard for comparing the masses of different atoms and calculating their relative atomic masses.
By defining the atomic mass of carbon-12 as exactly 12 AMU, scientists can determine the relative masses of other atoms and isotopes by comparing them to carbon-12. It allows for the determination of atomic masses on a relative scale, which is crucial for various scientific calculations and measurements in chemistry and physics.