Mass is the quantity of matter that a physical body contains. In daily life, we measure almost all things 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 kilogram, which is now defined by the Planck constant. Prior to Nov 2018, the kilogram was defined by the mass of a platinum alloy cylinder called the international prototype kilogram (IPK) that was 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 with reference to that of another atom or isotope, just as mass of daily objects were measured relative to that of the IPK. The question then is which isotope to choose as the reference mass and why.