Interatomic Forces

-The inter-atomic forces are electrical in nature.
- Such forces are active if the distance between the atoms is of the order of atomic size   (10^-10 m)

- An atom consists of a positively charged nucleus around which the negatively charged electrons resolve in certain definite orbitals.
- The positive charge on the nucleus if an atom is equal to the negative charge due to electrons.
- Hence the atom is electrically neutral.

- The inter-atomic force increases as the distance between atoms decreases.
- When the distance between two atoms is very small, then the electrons clouds of two atoms start overlapping and the inner atomic force become repulsive.
- The variation of interatomic force with distance is shown below:

-The interatomic force is equal to the negative gradient of the corresponding potential energy function.
F = - dU(r)/dr

Intermolecular Forces

- The force which is responsible to hold together the atom or molecules of a matter is called intermolecular force.
- When two molecules are far from each other, the force between them is attractive in nature and negligible.
- It has been found that the intermolecular force of attraction is inversely proportional to the seventh power of the distance between the molecules.

F_a ∝ 1/r⁷
or, F_a = - A/r⁷

where A is constant related to the nature of molecules and negative sign indicates that the force is attractive in nature.
As the distance between the molecules is decreased and made equal to the order of the dimension of the molecule, they began to repel each other.
- The force of repulsion varies inversely as the ninth power of the intermolecular separation.

F_r ∝ 1/r⁹
or, F_r ∝ B/r⁹ where B is constant related to the nature of the molecule.

Thus the net force acting on a molecule is given by
F = A/r⁷ + B/r⁹ 

-There is a definite distance r_o  between the molecules at which the force of attraction ad repulsion balance and the molecules are in the state of stable equilibrium.
i.e.,
F = 0 at r = r_o
r_o = √(B/A)