A molecule is the smallest particle of an element or a compound which can exist in a free state independently under normal conditions and exhibits all the properties of a substance (element or compound).

Mass of one mole of any substance is called its molar mass. Alternatively, the average mass of one mole of any substance is called its molar mass.

The quantity of a substance which contains Avogadro's number( 6 . 022 × 10 23 ) of chemical units (atoms, molecules or ions) of the substance is called a mole. The SI symbol of this is mol .

Atomicity: The number of all the atoms present in a molecule of a substance is termed as its atomicity. Based on the atomicity molecules are classified into mono, di, tri, tetra and polyatomic molecules. For example, each molecule of oxygen( O 2 ) is composed of two oxygen atoms. So, the atomicity of oxygen is two.