Atom is the smallest particle of an element, which may or may not have free existence and is capable of taking part in a chemical reaction. Atoms, as such, rarely occur in nature. On the other hand, a molecule is the smallest particle of a chemical compound and is capable of free existence. The molecules are, in fact, composed of the atoms. Most of the materials on the earth, however, are composed of different kinds of atoms in the form of chemical compounds.
Atoms combine to give rise to the formation of molecules. When the atoms of the same element combine, we get a molecule of that element. For instance, two atoms of oxygen combine to form a molecule of oxygen represented by O2. When the atoms of different elements combine, a molecule of a compound is formed. For example- one atom of hydrogen and one atom of chlorine combine to form a molecule of hydrogen chloride expressed as HCl.
Evidently, the molecules are formed by a chemical combination between the atoms. In a molecule, the atoms are not free; they do not have an independent existence but they are, in reality, chemically combined. In the case of the oxygen molecule, the two oxygen atoms are firmly bound to each other. Similarly, in hydrogen chloride molecule, the hydrogen and the chlorine atoms are firmly bound to each other. Thus, when atoms of the elements combine to form molecules, a force of attraction is developed between the atoms which hold them together in a molecule or in a chemical compound. This binding force between the atoms in a molecule is referred to as a chemical bond. The attractive force which holds the constituent atoms together in a molecule or in a chemical compound is called chemical bond. A chemical bond may be defined as the binding force which holds together atoms in a molecule.