The unit of amount of substance is mole which contains 6 · 023 × 10²³ entities. These entities may be atoms, molecules, ions, electrons or any other entity. This number 6·023 × 10²³ is called Avogadro’s number and is symbolised as N.

A mole is defined as the amount of any substance which contains Avogadro’s number (6·023 × 10²³) of particles (atoms, molecules or formula units) of that substance and has a mass equal to gram atomic mass, gram molecular mass or gram formula mass as the case may be e.g.,
               1 mole of oxygen atom
                                 = 6.023 x 10²³ atoms = 16 g
                1 mole of oxygen molecules
                                 = 6.023 x 10²³ molecules = 32 g
                1 mole of water molecules
                                  = 6.023 x 10²³ molecules = 18 g
                1 mole of sodium chloride
                                  = 6.023 x 10²³ formula units
                                   = 58.5 g
                 1 mole of electrons = 6.023 x 10²³ electrons

Mole in terms of volume: In the case of gases, it has been found that the volume occupied by one mole of any gas at STP (0°C and 1-atmosphere pressure) is 22·4 litres. The volume occupied by 1 mole of gas at STP is known as gram molecular volume (G.M.V.) or molar volume.
1 mole of gas  = 22.4 litres of gas at STP
For example,
1 mole of H₂ = 22.4 litres of H₂ at STP
1 mole of O₂ = 22.4 litres of O₂ at STP
Thus,
1 mole = 6.023 x 10²³ particles (i.e. atoms, molecules, formula units, ions, etc.)
           = Gram atomic mass (gram atoms), 
        gram molecular mass (gram molecules), gram formula mass, etc.
             = 22.4 litres at STP in case of gases.