In chemistry, a mole is a certain number of particles, usually of atoms or molecules. In theory, one could use any number of different terms for counting particles in chemistry. For example, one could talk about a dozen (12) particles or a gross (144) of particles. The problem with these terms is that they describe far fewer particles than one usually encounters in chemistry. Even the tiniest speck of sodium chloride (table salt), for example, contains trillions and trillions of particles.

The term mole, by contrast, refers to 6.022137 × 10 23 particles. Written out in the long form, it's 602,213,700,000,000,000,000,000 particles. This number is very special in chemistry and is given the name Avogadro's number, in honor of Italian chemist and physicist Amadeo Avogadro (1776–1856), who first suggested the concept of a molecule.

A unit like the mole (abbreviated mol) is needed because of the way chemists work with and think about matter. When chemists work in the laboratory, they typically handle a few grams of a substance. They might mix 15 grams of sodium with 15 grams of chlorine. But when substances react with each other, they don't do so by weight. That is, one gram of sodium does not react exactly with one gram of chlorine.

Instead, substances react with each other atom-by-atom or molecule-by-molecule. In the above example, one atom of sodium combines with one atom of chlorine. This ratio is not the same as the weight ratio because one atom of sodium weighs only half as much as one atom of chlorine.

The mole unit, then, acts as a bridge between the level on which chemists actually work in the laboratory (by weight, in grams) and the way substances actually react with each other (by individual particles, such as atoms). One mole of any substance—no matter what substance it is—always contains the same number of particles: the Avogadro number of particles.

Think of what this means in the reaction between sodium and chlorine. If a chemist wants this reaction to occur completely, then exactly the same number of particles of each must be added to the mixture. That is, the same number of moles of each must be used. One can say: 1 mole of sodium will react completely with 1 mole of chlorine. It's easy to calculate a mole of sodium; it is the atomic weight of sodium (22.98977) expressed in grams. And it's easy to calculate a mole of chlorine; it is the molecular weight of chlorine (70.906) expressed in grams. This conversion allows the chemist to weigh out exactly the right amount of sodium and chlorine to make sure the reaction between the two elements goes to completion.

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