posted by Blah .
what is an atomic mass unit? what are the approximate masses of the subtomic particles, expressed in atomic mass units?
1 amu is equal to one-twelfth of the mass of a carbon-12 atom, including its electrons. It is slightly less than the mass of a free neutron or a free proton.
A free electron has a mass of 1/1823 amu.
The mass of an atom is always less than the sum of the masses of its separate neutrons, protons and electrons, because the negative binding energy of the atom (which holds it together) reduces the mass.