“A chemical bond is defined as a force that holds two or more atoms together as a stable molecule”. Electrons play a key role in chemical bonding.
The attractive forces exist between atoms. The molecules of chemical substances are made of two or more atoms joined together by some force, among atoms. The forces that hold atoms together in stable molecules are referred to as chemical bond.
Electronic theory of Chemical Bonding
G.N. Lewis and W. Kossel, working independently, used the’ knowledge of electronic configuration to explain why atoms joined to form molecules. They visualised those noble gas atoms had a stable electronic configuration, ns2np6, except He which
is ls2, while atoms of all other elements had unstable or incomplete valence shell electronic configuration. All atoms have a tendency to achieve a stable noble gas electronic configuration.
In 1916, they put forward the electronic theory of chemical bonding. It states that in chemical bond formation atoms interact by losing, gaining or sharing of electrons so as to achieve a stable noble gas configuration. Thus molecules are formed by redistribution of electrons between two or more atoms.
We say that atoms react to acquire two electrons, ‘duplet’ configuration or eight electrons ‘octet’ structure. H, Li and Be atoms tend to acquire a duplet and all other atoms tend to attain the ‘octet’ configuration.
Energetics of Chemical Bond Formation
When atoms combine to form molecules, they come closer to each other and interact in such a way that the potential energy of the system is lowered (made more negative) due to the attractive forces. The lower the potential energy of a system, the more stable it is.
When the potential energy is lowered by 40 kJ/mol or more, we say that atoms are held together by a chemical bond. In order to explain the decrease in energy of the system in bond formation, a graph is plotted between the potential energy of two atoms against the intermolecular distance ‘R’ as shown in Fig. 2.1.
When two atom- approach each other, one possibility is that they repel each other. The repulsion increases with the decrease of distance. This will result in an increase in the potential energy of the system as shown by curve B.
The system will be unstable and the atoms will tend to move away from each other toward an infinite separation to attain lower potential energy. The other possibility is that the two atoms attract each other as shown by the curve A. The potential energy of the system decreases with the decrease in the distance between them.
However, after a certain limit, the forces of repulsion again begin to dominate resulting in a sharp increase of the potential energy.
The potential energy is minimum at an equilibrium bond distance ‘Re‘ where the two atoms are held together very firmly with the maximum possible decrease in energy.
At this ‘Re‘ the molecule attains maximum stability and a chemical bond is formed. At distances shorter or longer than ‘Re‘ the energy will be higher and the molecule will be unstable.
Types of Chemical Bonds
There are five types of bonds recognized by chemists: