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Intermolecular Forces

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Intermolecular Forces - Lesson Summary

Intermolecular forces are often called Van der Waals forces Van der waals forces can be classified into dispersion or London forces, dipole-dipole forces and dipole-induced dipole forces.

Atoms and non polar molecules have no dipole moment because symmetrical distribution of electron cloud.

Due to the constant motion of the electrons, an atom or molecule can develop an instantaneous or transient dipole when its electron cloud is distributed asymmetrically about its nucleus. The force of attraction between the two temporary dipoles is known as dispersion force or London force. This force is operates only at a short distance of 500 picometers and inversely proportional to distance between the two molecules.

The dipole - dipole forces are attractive forces that act between the polar molecules with permanent dipole.

The ends of the dipoles in a polar molecule possess partial charges, attraction between the positive end of one polar molecule and the negative end of the other polar molecule forms the dipole - dipole forces.

The more polar the molecule, the greater is the strength of its dipole- dipole interaction. Due to this, the boiling point of polar covalent compounds is relatively higher than that of non-polar covalent compounds.

Dipole-dipole forces are stronger compared to the other London forces. But due to the involvement of only partial charges they are weaker than the ion-ion interactions. The dipole-dipole interaction energy between the polar molecules is inversely proportional to the distance between the two polar molecules.

Polar molecules can interact both by dipole-dipole forces and also by London forces of attraction.

The attractive forces operate when a polar molecule with permanent dipole induces a dipole in a non polar atom or molecule by deforming its electron cloud is known as dipole-induced dipole forces.

Ex: Force of attraction existing between Chlorine and Water molecules

The induced dipole moment depends upon the dipole moment present in the permanent dipole and the polarisability of the non-polar molecule. Larger the size of the non-polar molecule higher is the polarisability and hence greater is the strength of the attractive interactions.

The intermolecular force is stronger than the Van der waal's forces of attraction. Intermolecular forces can also be repulsive in nature. The magnitude of repulsion increases rapidly with the decrease in the distance between the molecules. The physical state of a substance not only depends on the strength of intermolecular forces that exist between its atoms or molecules but also on the thermal energy of atoms or molecules.

The energy of a body arising from the translational, rotational or vibrational motion of its atoms or molecules is called the thermal energy. It is the measure of the average kinetic energy of the constituents of the matter and is directly proportional to temperature of the substance.

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