Complex sentence: The atomic mass ("m" a) is the mass of an atomic particle, sub-atomic particle, or molecule, most often expressed in unified atomic mass units, which by International agreement is 1/12 mass of a single carbon-12 atom at rest.
Split into simpler sentences: The atomic mass ("m" a) is the mass of an atomic particle, sub-atomic particle, or molecule. It may be expressed in unified atomic mass units, which by International agreement is 1/12 mass of a single carbon-12 atom at rest.