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'''Atomic mass''' (also commonly called atomic weight) is the average mass of an element's isotopes in [[Atomic mass units|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 [[proton]]s and [[neutron]]s 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=mc<sup>2</sup>.
*The atomic mass also takes into account the mass of the [[electron]]s 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.
[[Category:Chemistry]]
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.)
==See also==
*[[Mass number]]
[[Category:Physics]]
[[Category:Chemical Properties]]