The
abundance of a chemical element is a measure of the occurrence of the
element relative to all other elements in a given environment. Abundance is measured in one of three ways: by the
mass-fraction (the same as weight fraction); by the
mole-fraction (fraction of atoms by numerical count, or sometimes fraction of molecules in gases); or by the
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.