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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