Avogadro's constant

The SI definition of Avogadro's constant (designated N_A) is: the number of entities (such as atoms, ions, or molecules) per mole. (This definition requires a definition of mole that does not rely on N_A, but one that is in terms of ¹²C atoms). In the present definition N_A has dimension mol⁻¹. The numeric value of Avogadro's constant is N_A = 6.022 141 79 x 10²³ mol⁻¹.

Because the mole and Avogadro's number are defined in terms of the atomic mass constant (one twelfth of the mass of a ¹²C atom), Avogadro's constant and number have by definition the same numerical value. In practice the two terms are used interchangeably.

See for more details, about the history of the determination of the constant, etc. Avogadro's number.