The reason why the mass of an atom is expressed in atomic mass units (amu) rather than in grams is because atomic mass units make it easier to compare the masses of different atoms. This is because atoms of different elements have significantly different masses, making it impractical to express them in grams. Additionally, the use of atomic mass units allows for precise differentiation of the mass differences between isotopes of the same element. Therefore, expressing atomic mass in grams is not used because of the convenience and practicality offered by atomic mass units.
