Relative isotopic mass is defined as what?

Relative isotopic mass is defined as the mass of a specific isotope compared to 1/12 of the mass of an atom of carbon-12. This definition establishes a standard reference point for measuring the masses of isotopes. The carbon-12 isotope is commonly used for this purpose because of its stability and prevalence, allowing scientists to express the masses of other isotopes in relation to it.

This method of measurement is crucial in understanding the properties and behavior of different isotopes, which can have significant implications in various fields, including nuclear chemistry and isotopic labeling in research. Each isotope of an element may have a different mass due to variations in the number of neutrons, and relative isotopic mass helps quantify these differences effectively using the established standard.

Contextually, the other options do not accurately capture the definition of relative isotopic mass: the average mass of all isotopes would refer to isotopic abundance rather than individual isotopes; the mass of an atom divided by its atomic number does not yield a meaningful quantity related to isotopic mass; and the total mass of all electrons in an atom is unrelated to the concept of isotopic mass, which focuses on the nucleus's constituents rather than electrons.