Mole: Measuring the amount of substance
The mole measures amount of substance, with a fixed number of specified entities rather than a fixed mass. Its symbol is mol, and it is a base unit of the International System of Units. One mole contains exactly 6.02214076 times 10 to the power of 23 elementary entities. Those may be atoms, molecules, ions, ion pairs, or other particles. Like a dozen, the grouping connects a name to a collection of objects. The number is enormous because atoms are so small that ordinary laboratory samples contain vast collections, far beyond a few trillion entities.
Specifying the entity matters. A sample measured in moles may refer to individual atoms in an element or molecules in a compound. For example, equal amounts in moles of water and mercury contain equal numbers of their respective particles, with one mercury atom corresponding to each water molecule in the count. Their masses and volumes differ. For a crystal such as diamond, where the whole crystal can be treated as essentially one molecule, the counted entities are atoms bound together. In other solids, entities occupy a lattice but retain chemical identities relevant to the counting convention.
Chemical reactions use amount of substance to express proportions between reactants and products. The reaction between molecular hydrogen and molecular oxygen provides an example: two moles of hydrogen react with one mole of oxygen to form two moles of water. Those proportions describe entities participating in the reaction rather than equal masses of the materials. Solutions use a related quantity, molar concentration, which expresses dissolved substance per unit volume of solution. Moles per liter are a common unit for that concentration. The mole thus connects particle groupings to both reaction proportions and the composition of solutions handled in a laboratory.
The Avogadro number names the dimensionless count associated with one mole. The Avogadro constant has the same numerical value but carries the unit reciprocal mole. Dividing the number of entities by that constant gives the amount of substance. Before the present definition, the constant required experimental determination; measurements included counting silicon atoms in a single crystal. The earlier mole definition referred to the number of atoms in 12 grams of carbon-12. Under that arrangement, a substance's molar mass in grams per mole matched its average atomic or molecular mass expressed in daltons numerically, tying a laboratory quantity to an atomic mass scale.
Historical mass standards developed before the current amount of substance definition. John Dalton published a table of standard atomic weights in 1805, setting hydrogen's relative atomic mass to one. Jons Jacob Berzelius improved the precision of relative atomic masses and used oxygen as a reference, and his oxygen = 100 scale was widely used for decades. Later chemists helped resolve uncertainties about compound proportions. The word mole followed: Wilhelm Ostwald coined the German Mol from Molekul, meaning molecule, and an English translation appeared in 1897.
Standardization passed through oxygen-based references before a carbon-based definition emerged during the 1960s. The mole became the seventh SI base unit in 1971. In 2011, international measurement authorities agreed to plan a possible revision of the SI definitions. Scientists from more than 60 countries met in Versailles in November 2018, and definitions based on physical constants replaced definitions tied to physical objects. The changes took effect on 20 May 2019. Fixing the number of entities changed the older exact relationship between molar mass and atomic mass into an extremely close approximation, while making the entity count itself exact.
Engineering scales introduce larger groupings. A kilomole is 1000 moles, matching the SI pattern of metric prefixes. Paired with cubic meters, it preserves the numerical concentration obtained from moles per liter, because both the substance quantity and volume scale together. Chemical engineers formerly used kilogram-mole and gram-mole terminology tied to carbon-12 reference masses. Some also adopted the pound-mole for calculations in customary units, defining it through the number of entities in 12 pounds of carbon-12. A different derived unit, the katal, expresses catalytic activity as one mole per second. These variations connect the underlying grouping to different quantities and working scales.
At much smaller scales, the naming system reaches below a whole entity. One femtomole corresponds to exactly 602214076 molecules, while attomoles and smaller quantities do not necessarily correspond to whole numbers. The yoctomole is approximately 0.6 of an individual molecule and appeared in scientific journals in the year its prefix was formally introduced. Elsewhere, chemists mark Mole Day on October 23, using the American date notation 10/23 to echo the constant's power of ten. Celebrations can run from 6:02 in the morning to 6:02 in the evening, with alternative dates drawing on other parts of the number.