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.
5.2 FORMATION OF CHEMICAL BONDS.
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.
5.3 TYPES OF CHEMICAL BONDS.
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.
The ionic or electrovalent bond (by the transfer
of one or more electrons from one atom to another, to
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.