Whether we weigh a substance or measure the volume of a gas, any sample of matter we examine contains particles called molecules which in turn are composed of atoms. Thus, we never work with individual atoms or molecules but always with the aggregates of these.
Now, it is a common practice to count the particles in convenient numbers. For example- we count oranges in dozen, money in rupees, paper sheets by reams and so on. Chemists, therefore, have selected a unit much larger than a single atom or molecule for the purpose of comparison of the amounts of various materials. To a chemist, this unit, termed as a mole, means a definite number of individual unit particles of whatever kind of material is being considered. During the nineteenth century, it was decided by the chemists to take the number of molecules in a sample of oxygen weighing 32 grams as a standard number. A mole, therefore, was defined as the number of oxygen molecules in exactly 32 grams of oxygen. Mole is identified with this number of particles rather than weight. The number determined later to be 6.023 X 1023 is called Avogadro’s number (N). Thus, a mole is Avogadro’s number of chemical units under consideration. Hence, the number of particles present in a mole is 6.023 X 1023. For example, a mole of chlorine atoms includes 6.023 X 1023 atoms of chlorine and a mole of chlorine molecules includes 6.023 X 1023 molecules or 2 X6.023 X 1023 atoms of chlorine because each chlorine molecule contains two atoms.
Significance of a Mole:
The significance of a mole is that it refers to both number (6.023 X 1023) as well as the mass in grams, numerically equal to the molecular, atomic or ionic mass in atomic mass units. For example-
(1) One mole of oxygen signifies-
6.023 X 1023 molecules of oxygen or 2 X6.023 X 1023 atoms of oxygen as each oxygen molecule, O2, is diatomic.
32 grams of oxygen (because the molecular mass of oxygen is 32 a.m.u).
(2) One mole of water signifies-
6.023 X 1023 molecules of water.
18.016 or 18 grams of water (because the molecular mass of water is 18.016 or 18 a.m.u).