Kinetic molecular interpretation of solids

Kinetic molecular interpretation of solids

Kinetically the crystalline solids can be interpreted as follows:

Attractive Forces:

The attractive forces among the molecules are maximum due to the closest packing of the molecules.


The molecules in solids are closely packed. Therefore their movement is restricted (limited). As a result, they are rigid in nature. The molecules cannot move due to maximum attractive forces between them. However, molecules vibrate about their mean position.

High density:

The molecules of a crystalline solid are closely packed. As a result, molecules of solid occupy minimum volume. As density is inversely proportional to volume, therefore high density will be observed due to the minimum volume existing between the molecules.


As there is no translational movement of particles in a solid, therefore, there are no collisions among the molecules.

Kinetic Energy:

There is negligible translational and rotational kinetic energy in solid molecules. However, they can vibrate about their mean positions. So molecules of solid possess only vibrational K.E.

Geometric Shape:

The crystalline solids have definite distinctive geometrical shapes. It is due to the definite and orderly arrangement of atoms, ions or molecules in a three-dimensional shape.

Properties Of Solids:


The diffusion depends upon the velocity of molecules. As the movement of the molecules is very slow. therefore, the diffusion will be minimum. Had there been diffusion in solids? The centuries-old buildings would have collapsed in no time.

Compression (Effect of Pressure):

There is practically no effect of pressure on solids as the molecules are very closely packed together.
The effect of pressure on solids is expressed in terms of compressibility (β). This is characterized as, “The diminishing in volume per unit volume when the weight is expanded by one environment.


The coefficient of expansion (α) is defined as ” the increase in volume per unit volume when the temperature is increased by 1 ℃.” When a solid is heated, its geometric shape changes until at a certain temperature, it changes into the liquid form. The temperature at which a solid, changes into the liquid form is called the melting point of a solid.”

The motion of Molecules:

There is no translational and rotational motion due to the presence of strong intermolecular forces in the crystalline solid, as the molecules are already closely packed together However the molecules can vibrate about their mean position.

Intermolecular Forces:

In solids, the intermolecular forces are maximum in between the particles. These are held together in fixed positions by a strong attractive force. They can vibrate only about their fixed position.

Kinetic Energy Based upon Kinetic Molecular Theory:

According to the kinetic molecular Theory, the attractive forces between the solid particle are maximum. This is due to the minimum distance between them and, therefore, molecules do not possess translation and vibrational kinetic energies. However, they can vibrate about their mean position. So they possess vibrational kinetic energy.


There are two types of solids:

  1. Amorphous Solids: The solids which have no definite geometric shape are called  amorphous solids e.g. glass, rubber, dust, etc,
  2. Crystalline Solids: The solid, Which has a definite regular and three-dimensional geometric shape are called crystallization e.g. CuSO4. 5H2O   Na2SO4. 10H2O.

Differences between Amorphous and crystalline Solids:

  1. In the crystalline state, The crystals possess a definite geometrical structure e.g. NaCl is cubic in nature. In this amorphous state, the solid does not have a definite geometrical structure e.g. glass.
  2. There is a complete regularity of arrangement of atoms, ions or molecules in a crystalline solid whereas the atoms, ions or molecules are not arranged in a regular manner in amorphous solids.
  3. The crystalline substance has a sharp melting point while the amorphous solid does not have a sharp melting point and gradually softens on heating. They may be called as super-cooled liquids e.g. glass, plastic, etc.
  4. In a crystalline substance, water molecules are a part of crystal e.g. FeSO4. 7H2O, CuSO4. 5H2O etc while the amorphous substances have no such water of crystallization.
  5. In some cases, the color of a crystalline substance is due to the water of crystallization e,g, CuSO4. 5H20 is blue. The color of crystals fades away in CuSO4. 5H20 when it is heated.
  6. The amorphous solid is a liquid is a solid which softens on heating ultimately they become mobile over a wide range of temperatures while the crystalline substances have no such property.

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