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 the mass-fraction (the same as weight fraction), or mole-fraction (fraction of atoms, or sometimes fraction of molecules, in gases), or by volume fraction. Measurement by volume-fraction is a common abundance measure in mixed gases such as atmospheres, which is close to molecular mole-fraction for ideal gas mixtures (i.e., gas mixtures at relatively low densities and pressures). For example, the mass-fraction abundance of oxygen in water is about 89%, because that is the fraction of water's mass which is oxygen. However, the mole-fraction abundance of oxygen in water is only 33% because only 1 atom in 3 in water is an oxygen atom. In the universe as a whole, and in the atmospheres of gas-giant planets such as Jupiter, the mass-fraction abundances of hydrogen and helium are about 74% and 23-25% respectively, while the (atomic) mole-fractions of these elements are closer to 92% and 8%.

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