Question

Elaborate on the concept of elasticity at its connection with interatomic forces. Be as detailed as possible.

Answer 1

The forces between the atoms due to electrostatic interaction between the charges of the atoms are called
interatomic forces. These forces are electrical in nature and these are active if the distance between the two
atoms is of the order of atomic size i.e. 10^–10 metre.

(1) Every atom is electrically neutral, the number of electrons (negative charge) orbiting around the
nucleus is equal to the number of proton (positive charge) in the nucleus. So if two atoms are placed at a very
large distance from each other then there will be a very small (negligible) interatomic force working between
them.

(2) When these two atoms are brought close to each other to a distance of the order of 10^–10 m, the
distances between their positive nuclei and negative electron clouds get disturbed, and due to this, attractive
interatomic force is produced between two atoms.

(3) This attractive force increases continuously with decrease in r and becomes maximum for one value of
r called critical distance. Beyond this the attractive force starts decreasing rapidly with further decrease in the value of r.

(4) When the distance between the two atoms becomes r₀, the interatomic
force will be zero. This distance r₀ is called normal or equilibrium distance.
(r₀ = 0.74 Å for hydrogen).

(5) When the distance between the two atoms further decreased, the
interatomic force becomes repulsive in nature and increases very rapidly with
decrease in distance between two atoms.

6)The potential energy U is related with the interatomic force F by the following relation.

F=dU/dr

(i) When two atoms are at very large distance, the potential energy is negative and becomes more negative as r is
decreased.
(ii) When the distance between the two atoms becomes r0, the potential energy of the system of two atoms
becomes minimum (i.e. attains maximum negative value). As the state of minimum potential energy is the state
of equilibrium, hence the two atoms at separation r0 will be in a state of equilibrium.

( U₀₌ 7.2 x10^-19 Joule for hydrogen).

(iii) When the distance between the two atoms is further decreased (i.e. r < r0) the negative value of
potential energy of the system starts decreasing. It becomes zero and then attains positive value with further
decrease in r