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Abundance of the chemical elements: The abundance of the chemical elements is a measure of the occurrence of the chemical elements relative to all other elements in a given environment. Abundance is measured in one of three ways: by the mass-fraction ; by the mole-fraction (fraction of atoms by numerical count, or sometimes fraction of molecules in gases); or by the volume-fraction.
Abundance of elements in Earth's crust: The abundance of elements in Earth's crust is shown in tabulated form with the estimated crustal abundance for each chemical element shown as mg/kg, or parts per million by mass (10,000 ppm = 1%). Note that the noble gases are not included, as they form no part of the solid crust.
Abundance (chemistry): In a chemical reaction, a reactant is considered to be in abundance if the quantity of that substance is high and virtually unchanged by the reaction. Abundance differs from excess in that a reactant in excess is simply any reactant other than the limiting reagent; the amount by which a reactant is in excess is often specified, such as with terms like "twofold excess", indicating that there is twice the amount of reactant necessary for the limiting reagent to be completely reacted.