As we examine the world around us, we find that it is generally made up of compounds and mixtures. Rocks, coal, soil, air, trees, animals, all are formed by combination of atoms. Substances composed of single atoms are very rare. Examples are, argon (Ar) in the atmosphere and helium (He), mixed with natural gas. Clearly, there must be some force that holds atoms together in a molecule or crystals, otherwise the atoms would simply fly apart, and no compound could exist. The force which holds atoms together in a molecule or a crystal is called a chemical bond.


Before the discovery of electrical structure of atoms, the nature of the forces holding the atoms together in a molecule or crystal was a mystery. Now it is believed that these forces are electrical in nature and that the chemical reactions that occur between atoms involve change in their electronic structures. The electrons in the outer most shell of an atom are called the valence electrons. In the formation of chemical compounds from the elements, the valence electrons are either transferred from the outer shell of one atom to the outer shell of another atom or shared between them. This produces a chemical bond. When an atom of one element chemically combines with the atom of another element, both atoms usually attain a stable outer shell, consisting of eight electrons (octet). Only hydrogen H2 and helium (He) atom have the stable outer shell of two electrons (Duplet). This is in accordance with the general rule that all processes tend to move towards the state of maximum stability. Generally, a stable molecule occurs, when the total energy of the combined atoms, is less than the total energy of the individual atoms.


The first explanations of the nature of chemical bonds were advanced by W. Kossel (a German scientist) and G. N. Lewis (an American chemist) in 1916; they proposed two major types of chemicals bonds.

1.    The ionic or electrovalent bond (by the transfer of one or more electrons from one atom to another, to form ions).

2.    The covalent bond (a bond that results when atoms share electrons). Other types of bond include metallic bonds and hydrogen bonding.


Chemical bonding also plays a role in determining the state of matter. At room temperature, water is liquid, carbon dioxide (CO2) is a gas and table salt, sodium chloride (NaCl) is solid, because of difference in chemical bonding.

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