Definitions for "Abundance of the chemical elements"
The abundance of a chemical element measures how relatively common the element is, or how much of the element there is by comparison to all other elements. Abundance may be variously measured by mass-fraction (the same as weight fraction), or mole-fraction (fraction of atoms), or by volume fraction. Measurement by volume-fraction is a common abundance measure in mixed gases such as atmospheres, which is close to mole-fraction for ideal gas mixtures (i.e., gas mixtures at relatively low densities and pressures).