Atomic mass (also commonly called atomic weight) is the average mass of an element's isotopes in amu (atomic mass units). This is roughly equal to the mass number, but takes into account some subtle issues. The mass number is simply the sum of the protons and neutrons in a given atom. Protons and neutrons have very nearly the same mass, and the amu is defined to be roughly that amount. This means that atomic weights are nearly integers, but there are a few effects making this not quite true:
- The atoms of a given element are often a mixture of different isotopes, that is, elements with different numbers of neutrons. The agreed-upon atomic mass of an element is the average of the masses of its isotopes, weighted by their observed abundance on Earth.
- The mass of a nucleus is not precisely the sum of the masses of its constituents, because of the binding energy and the relation E=mc2.
- The atomic mass also takes into account the mass of the electrons circling the nucleus. While the electrons weigh only about 1/2000th as much as the protons, this makes a difference. Atomic mass is not the same as nuclear mass.
An element's atomic mass is found below its symbol on the periodic table of the elements.
The value of the amu is actually defined to give carbon-12 a mass of exactly 12. (Note that the atomic mass listed for carbon in the periodic table is 12.0107; this is because 1.07% of the carbon in nature is carbon-13.)