A mole is a counting unit. Just as a dozen means 12, a mole means 6.022 times 10 to the 23. That is Avogadro's number. It is the number of atoms in 12 grams of carbon-12. It is also the number of molecules in 18 grams of water or 180 grams of glucose. The mole is the bridge between the microscopic world of atoms and the macroscopic world of grams.

The mole is one of the seven base units of the International System of Units. It was redefined in 2019 as exactly 6.02214076 times 10 to the 23 entities. The entities can be atoms, molecules, ions, electrons, or anything else. The definition fixes the number, which makes the mole independent of any physical artifact.

The mole is used throughout chemistry. Molar mass is the mass of one mole of a substance. Molarity is moles of solute per liter of solution. Stoichiometry uses moles to relate reactants and products. The ideal gas law uses moles to relate pressure, volume, and temperature. The mole is the central unit of quantitative chemistry.

The number is enormous. One mole of marbles would cover the Earth to a depth of several kilometers. One mole of water is about 18 milliliters, a little more than a tablespoon. The mole is large because atoms are small. It is the scale factor between the two worlds.