The number of elementary entities in one mole of a substance. A mole is defined as an amount of a substance that contains as many elementary entities as there are atoms in exactly 12 g of ¹²C; the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles. Experiments give 6.0221367 × 10²³ as the value of Avogadro's number. In most calculations the coefficient is rounded off to 6.02. Thus, a mole of ¹²C atoms has 6.02 × 10²³ carbon atoms, a mole of water molecules contains 6.02 × 10²³ H₂O molecules, a mole of electrons contains 6.02 × 10²³ electrons, and so forth.