Intermolecular or interatomic forces
Consider two isolated hydrogen atoms moving towards each other as shown in Fig..
As they approach each other, the following interactions are observed.
1. Attractive force A between the nucleus of one atom and electron of the other. This attractive force tends to decrease the potential energy of the atomic system.
2. Repulsive force R between the nucleus of one atom and the nucleus of the other atom and electron of one atom with the electron of the other atom. These repulsive forces always tend to increase the energy of the atomic system.
There is a universal tendency of all systems to acquire a state of minimum potential energy. This stage of minimum potential energy corresponds to maximum stability.
If the net effect of the forces of attraction and repulsion leads to decrease in the energy of the system, the two atoms come closer to each other and form a covalent bond by sharing of electrons. On the other hand, if the repulsive forces are more and there is increase in the energy of the system, the atoms will repel each other and do not form a bond.
The variation of potential energy with interatomic distance between the atoms is shown in Fig..
It is evident from the graph that as the atoms come closer i.e. when the interatomic distance between them decreases, a stage is reached when the potential energy of the system decreases. When the two hydrogen atoms are sufficiently closer, sharing of electrons takes place between them and the potential energy is minimum. This results in the formation of covalent bond and the interatomic distance is ro.
In solids the interatomic distance is ro and in the case of liquids it is greater than ro. For gases, it is much greater than ro.
The forces acting between the atoms due to electrostatic interaction between the charges of the atoms are called interatomic forces. Thus, interatomic forces are electrical in nature. The interatomic forces are active if the distance between the two atoms is of the order of atomic size ≈ 10-10 m. In the case of molecules, the range of the force is of the order of 10-9 m.