Atoms rarely occur in the free state at ordinary temperature. Except noble gases, which are chemically unreactive, atoms of all other elements have a tendency to combine either with each other or with the atoms of other elements to form cluster or aggregates of atoms with definite composition. A cluster formed may be either a molecule or an ion. The attraction between atoms forming the cluster is called a chemical bond or valence bond. Thus:
A chemical bond is defined as the attractive force that holds two or more atoms together in a molecule or an ion.
Our main concern in this chapter is to consider the interactions between atoms in the light of the structure of atom and find out the answers of the following questions
(a) Why do atoms combine?
(b) How do atoms combine together?
(c) How can the properties of compounds be understood and determined in terms of the forces that keep the atoms together in molecules.
Why Do Atoms Combine? (Cause of Chemical Bonding)
By a close study of atoms and molecules it has been found that atoms combine chemically for the following reasons:
1. Net attractive force between atoms: Atoms consist of strongly positive nucleus and negative electrons. When two atoms come closer to combine with each other to form a bond between them, the attractive and repulsive forces begin to operate between them. The attractive forces are between the electrons of one atom and the nucleus of the other atom while the repulsive forces are between the electrons or the nuclei of the two atoms.
When the two atoms approach closer to each other, these forces counteract each other. The net result of these forces may be either attraction or repulsion between the atoms. If the attractive forces become dominant over the repulsive forces, the net result is the attraction between the atoms and hence they combine together to form a chemical bond between them. On the other hand if the repulsive forces become dominant over the attractive forces, the atoms do not combine and hence no chemical bond is established between them. For example in case of hydrogen atoms, the net result is attraction and hence two H-atoms combine together to form H₂ molecule. On the other hand in case of helium atoms, the net result is repulsion and hence two He-atoms do not combine together to form He₂ molecule.
2. Octet rule or rule of eight: (Electronic Theory of valency or octet theory of valency). Lewis, Kossel and Longmuir (1916) tried, for the first time, to explain why atoms combine together on the basis of the electronic configuration of noble gases as given below.
They assumed that since the atoms of noble gases do not normally react with other atoms to form compounds, it is reasonable to assume that the outermost shell configuration of the atoms of noble gases is a stable configuration of 8 electrons which they called an octet. They also concluded that the two electrons in case of helium (called duplet) is also as stable as an octet present in other noble gases. Since the octet of electrons is so stable in the gases, one can reasonably assume that when atoms of other elements combine to form a molecule, the electrons in their outermost orbits are arranged between themselves in such a way that they achieve an octet of electrons which is stable and thus a chemical bond is established between the atoms.
The tendency of the atoms to have eight electrons in their outermost shell is known as octet rule or rule of eight. Since helium atom has only two electrons, this rule is called doublet rule or rule of two in case of helium. Octet rule was given in the form of a theory which is known as octet theory of valency or electronic theory of valency which states that:
In the formation of a chemical bond atoms interact with each other by losing, gaining or sharing of electrons so as to acquire a stable outer-shell of eight electrons.
3. Lowering of energy of combining atoms: When two atoms combine together to form a bond, there is an over all decrease in the potential energy of the combining atoms, i.e., a system having bonded atoms has lower energy than that having the unbonded atoms. This implies that the system of bonded atoms having lower energy is more stable than that of unbonded atoms having higher energy. From this it, therefore, follows that the process of chemical bonding between the atoms decreases the energy of the combining atoms and gives rise to the formation of a system which has lower energy and hence has greater stability. Potential Energy Curve
The curve given in Fig. 7.2 shows the variation of potential energy with the distance between the nuclei of two atoms A and B which are approaching closer to each other to form a bond between them. The trend of the curve from right to left should be observed. When the two atoms A and B are far away, say at an infinite distance, from each other, the attraction between them is zero and hence, by convention, the energy of each of the atoms in arbitrarily taken to be zero and for stable system it is negative.