4.0 Introduction

Scientists are constantly discovering new compounds, orderly arranging the facts about them, trying to explain with the existing knowledge, organising to modify the earlier views or evolve theories to explain the newly observed facts.

Molecule

Matter is made up of one or different types of elements. Under normal conditions, no other element exists as an independent atom in nature, except noble gases. However, a group of atoms is found to exist together as one species having characteristic properties. Such a group of atoms is called a molecule.

Chemical Bond

There must be some force that holds these constituent atoms together in the molecules. The attraction force that holds various constituents (atoms, ions, etc) together in different chemical species is called a chemical bond.

Since the formation of chemical compounds takes place as a result of the combination of atoms of various elements in different ways, it raises many questions. Why do atoms combine? Why are only certain combinations possible? Why do some atoms combine while certain others do not? Why do molecules possess definite shapes? To answer such questions different theories and concepts have been put forward from time to time.

These are the Kössel-Lewis approach, the Valence Shell Electron Pair Repulsion (VSEPR) Theory, the Valence Bond (VB) Theory and the Molecular Orbital (MO) Theory. The evolution of various theories of valence and the interpretation of the nature of chemical bonds have closely been related to the developments in the understanding of the structure of atoms, the electronic configuration of elements and the periodic table. Every system tends to be more stable and bonding is nature’s way of lowering the energy of the system to attain stability.