What and how much is a mole?

The mole is the unit of for amount of substance. It is defined as the amount of a chemical substance that contains as many elementary entities, e.g., atoms, molecules, ions, electrons, or photons, as there are atoms in 12 grams of carbon-12. This number is expressed by the Avogadro constant, which has a value of 6.022×10^23 mol−1.