In chemistry, a mole is a certain number of particles, usually of atoms or molecules. Just as a dozen particles (abbreviated doz.) would be 12 particles, a mole of particles (abbreviated mol) is 6.022137 x 10²³ particles. This number, usually shortened to 6.02 x 10²³, is known as Avogadro's number in honor of Count Amedeo Avogadro, an Italian professor of chemistry and physics at the University of Turin who was the first person to distinguish in a useful way between atoms and molecules. It is such a huge number (more than 600 billion trillion) because atoms and molecules are so incredibly tiny that we must have huge numbers of them before we can do anything useful with them.

A standard unit for counting numbers of particles is needed in chemistry, because atoms and molecules react with one another particle by particle. The amount of a chemical reaction--how much of the chemicals are used up...