Mole is a link between the mass of atoms (or molecules) and the number of atoms (or molecules).
One gram atom of any element contains the same number of atoms and one gram molecule of any substance contains the same number of molecules.
This number has been experimentally determined and found to be equal to 6.022137 × 10^{23} correct up to 7 significant figures.
This value was given by Amedeo Avogadro and is called as Avogadro number. The value is generally used as 6.022 × 10^{23} denoted by N_{A} or N.
The amount of substance containing Avogadro’s number of atoms or molecules is called a mole.
A mole is chemists unit for counting entities at the microscopic level, such as atoms, molecules, ions, electrons, protons, neutrons, etc.
Thus, one mole is defined as the amount of a substance that contains as many entities or particles as there are atoms in exactly 12 g (or 0.012 kg) of the C^{12} isotope.