In imprecise terms, one AMU is the average of the proton rest mass and the neutron rest mass. This is approximately 1.67377 x 10 -27 kilogram (kg), or 1.67377 x 10 -24 gram (g). The mass of an atom in AMU is roughly equal to the sum of the number of protons and neutrons in the nucleus.
The AMU is used to express the relative masses of, and thereby differentiate between, various isotopes of elements. Thus, for example, uranium-235 (U-235) has an AMU of approximately 235, while uranium-238 (U-238) is slightly more massive. The difference results from the fact that U-238, the most abundant naturally occurring isotope of uranium, has three more neutrons than U-235, an isotope that has been used in nuclear reactors and atomic bombs.
Also see our Table of Physical Units and Constants.