Atomic and nuclear masses

The masses of atoms and subatomic particles are extremely small - for example the mass of an oxygen atom is about 3x10^{- 26} kg and that of a neutron about 1.67x10^-27 kg. It is therefore convenient to define a new unit to measure them.

This is known as the atomic mass unit – written as a.m.u or simply as u.

One atomic mass unit is defined as one twelfth of the mass of one atom of the carbon 12 isotope.

1 u = 1/12(19.92x10^-27)kg = 1.66x10^-27 kg

The masses of some other particles and atoms are given below:

Electron	0.000 548 u
Proton	1.007 273 u
Hydrogen atom	1.007 825 u
Neutron	1.008 665 u
Alpha particle	4.001 508 u
Helium 4 atom	4.002 604 u
Uranium 235 atom	235.172 7 u

Alternative units for energy and mass
You will have probably met an alternative unit for energy the MeV – related to the Joule by:

1 MeV = 10⁶ eV = 1.6x10^-19 J

But using Einstein's mass-energy relationship (E = mc²) we can convert the units for mass (kg) into alternative units. The units for mass being MeV/c².

Therefore: 1 u = 1.66x10^-27 kg = 931.5 MeV/c² = 1.54x10^-10 J

The proton therefore has a rest mass of 1.007 273 u or 938 MeV/c² and a rest energy of 938 MeV.