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Unified Mass Unit The unified mass unit (u) is also referred to as the atomic mass unit (amu). An amu is defined as exactly 1/12 the mass of one atom of carbon-12. A mole of anything is 6.022 x 1023 of those things and the atomic mass system is designed so that 1.000 mole of carbon-12 has a mass of 12.00 grams (notice that everything here is 4 sig figs). Particle Carbon-12 electron proton neutron Mass (u) 12.000000 0.000549 1.007276 1.008665 Mass (g/mol) 12.00000 0.000549 1.007276 1.008665 There is also an energy unit that is associated with the amu. The unit is the electron volt (eV). 1.60 x 10-19 J = 1.00 eV The electron volt is related to the charge on an electron and it turns out to be a convenient way to measure energy at the atomic level. The conversion of one amu of mass into energy produces 931.50 MeV (5 sig figs). In nuclear chemistry it is common to express mass in terms of MeV/c2. 1.0000 u = 931.50 MeV/c2 The masses and energy equivalents are shown below. Particle electron proton neutron Mass (u) 0.000549 1.007276 1.008665 Energy (MeV) 0.511 938.3 939.6