Van der Wall’s forces
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van der Waal’s forces
These are interparticle forces among the non-polar molecules such as hydrogen
(H2), oxygen (O2), chlorine (Cl2) in solid or liquid states. The existence of such
forces was first of all given by the Dutch chemist van der Waal and these are
known after his name as van der Waal’s forces.
Origin of van der Waal’s forces
In order to understand the
origin of van der Waal’s forces, let us consider a monatomic molecule of neon.
Each neon atom has its electron cloud symmetrically distributed around the
nucleus. It can be imagined that due to rapid movement of electrons, the
electron distribution of the atom may become unsymmetrical with slight
increase of electron density on one side. This would to the separation of positive
and negative ends at a particular instant. In other words, the molecule develops
a temporary dipole. Such an atom having a small temporary dipole is called
instantaneous dipole. This instantaneous dipole influences the electron
distribution of the other molecules in the close vicinity and induces dipole in
them also. The attractive forces between the instantaneous dipoles and induced
dipoles are called van der Waal’s forces. These are then attracted to each other
exactly in the same way as the permanent dipole.
Instantaneous dipole
due to motion of
electrons
Instantaneous
dipole
Symmetrical distribution of electron
cloud
Instantaneous induced dipole
Origin of van der waal’s forces
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Van der Wall’s forces
Free resources –A/AS level (1)
Molecular Structures of non metal elements and compounds When constituent
particles of a solid are molecules, these are called molecular solids. In molecular
solids the interparticle forces are dipole-dipole interactions or hydrogen
bonding or van der Waal’s forces.
The dipole- dipole interaction arises due to electrostatic interactions between
the oppositely charged ends of permanent dipoles.
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Dipole- dipole forces between polar molecules.
Example:
1. The molecules of HCl, CO2 are held together by Dipole- dipole
forces (Physical state are gas).
2. Napthalene (C10H8) and Iodine (I2) are the molecular solids in
which the intermolecular forces are dipole-dipole interaction and
van der Waal’s forces respectively.
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