A lot vs. Alot: 9 Grammatical Pitfalls
atomic mass n.
The mass of an atom, usually expressed in atomic mass units.
the quantity of matter contained in an atom of an element. It is expressed as a multiple of one-twelfth the mass of the carbon-12 atom, 1.992646481023gram, which is assigned an atomic mass of 12 units. In this scale 1 atomic mass unit (amu) corresponds to 1.660538731024gram.