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[[File:Universe content pie chart.jpg|thumb|200px|Estimated proportions of [[matter]], [[dark matter]] and [[dark energy]] in the [[universe]]. Only the [[fraction]] of the [[mass]] and [[energy]] in the [[universe]] labeled [[atom|"atoms"]] is composed of [[chemical elements]].]]
The '''abundance''' of a [[chemical element]] measures how common is the element relative to all other elements in a given environment. Abundance is measured in one of three ways: by the [[mass fraction (chemistry)|mass-fraction]] (the same as weight fraction); by the [[mole fraction|mole-fraction]] (fraction of atoms by numerical count, or sometimes fraction of molecules in gases); or by the [[volume fraction|volume-fraction]]. Volume-fraction is a common abundance measure in mixed gases such as planetary atmospheres, and is similar in value to molecular mole-fraction for gas mixtures at relatively low densities and pressures, and [[ideal gas]] mixtures. Most abundance values in this article are given as mass-fractions.
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