Mass is the quantity of matter that a physical body contains. In daily life, we measure almost everything using the concept of inertia mass, which is a measure of a body’s resistance to acceleration. Such measurements are made in relation to the kilogramme, which is now defined by the Planck constant. Prior to Nov 2018, the kilogramme was defined by the mass of a platinum alloy cylinder called the International Prototype Kilogramme (IPK), which is stored in France. The IPK contains octillions of atoms and has inevitably gained or lost mass over time through oxidation. Therefore, it cannot be used to accurately measure the mass of atoms.
To circumvent this problem, scientists decided to measure the mass of an atom or isotope relative to that of another atom or isotope, just as the mass of everyday objects was measured in relation to the IPK. The question then arises: which isotope should be chosen as the reference mass, and why?