Avogadro's number, or Avogadro's constant, is the number of particles found in one mole of a substance. It is the number of atoms in exactly 12 grams of carbon-12. This experimentally determined value is approximately 6.0221 x 10^{23} particles per mole. Avogadro's number may be designated using the symbol L or N_{A}. Note that Avogadro's number, on its own, is a dimensionless quantity.

In chemistry and physics, Avogadro's number usually refers to a quantity of atoms, molecules, or ions, but it can be applied to any "particle." For example, 6.02 x 10^{23} elephants is the number of elephants in one mole of them! Atoms, molecules, and ions are much less massive than elephants, so there needed to be a large number to refer to a uniform quantity of them so that they could be compared relative to each other in chemical equations and reactions.

### History of Avogadro's Number

Avogadro's number is named in honor of the Italian scientist Amedeo Avogadro. Although Avogadro proposed that the volume of a gas at a fixed temperature and pressure was proportional to the number of particles it contained, he did *not* propose the constant.

In 1909, French physicist Jean Perrin proposed Avogadro's number. He won the 1926 Nobel Prize in physics for using several methods to determine the value of the constant. However, Perrin's value was based on the number of atoms in 1 gram-molecule of atomic hydrogen. Later, the constant was redefined based on 12 grams of carbon-12. In German literature, the number is also called the Loschmidt constant.