There are two types of forces present in molecules, that
(1) Intra-molecular forces and
(2) Inter-molecular forces.
Intra-molecular forces hold atoms together in a
molecule. For example, - water (H2O) molecule
consists of two hydrogen atoms and one oxygen atom
joined together in a specific way, that we call covalent
Inter-molecular forces are the attractive forces between
the neutral molecules, which hold them together at
certain temperature. These attractive forces of neutral
molecules between each other are called “Vander Waal's
Forces", named after the Dutch Physicist (1837-1923),
who first described them. There are three types of
attractive forces between molecules (1) Dispersion
forces (2) Dipole-dipole forces, induced dipole and (3)
Generally, inter molecular forces are much weaker than
1. Dispersion Forces (London forces):
All particles, whether individual atoms, ions or
molecules experience dispersion forces, which result
from the motion of electrons around atom. For example
consider atoms of noble gases e.g. He, Ne, Ar. etc. Let
us examine the attractive forces in neon as an example.
The distribution of ten electrons around the nucleus of
neon is spherically symmetrical. But in case when two (Ne)
atoms, come extremely close together. The electron
clouds will repel each other. This polarizes each
molecule and gives rise to an induced or temporary
dipoles and as a result weak attractive forces called
dispersion forces also called London forces after Fritz
London (who first identified them in 1930) are
developed. The attraction is strong when particles are
close together but rapidly weakens as they move apart.
Induced or temporary dipoles in Neon molecules
Thus dispersion forces (London forces) are the weak
attractive forces between temporarily polarized atoms
(or molecules) caused by the varying positions of the
electrons during their motion about the nuclei.
London forces are generally small as their energies are
in the range of 1-10 KJ/mol.
2. Dipole-Dipole forces:
Dipole-dipole forces are forces that act between polar
molecules that possess dipole moments. A Dipole-dipole
force, is an attractive inter-molecular force resulting
from the inter action of the positive end of one
molecule with the negative end of other.
Dipole-Dipole forces are generally stronger than
Consider the (H-Cl) molecule, as chlorine has greater
electronegativity than hydrogen, a partial negative
charge on chlorine atom, and a partial positive charge
on hydrogen atom is developed. The (H+8—CI-8)
has a permanent dipole moment. The electrostatic
attraction of positive end of one (HC1) molecule for the
negative end of another constitutes attractive forces in
addition to dispersion forces. As a result polar (HQ)
boils at (- 85°C) but non-polar (F2) boils at
(- 188°C) though both have nearly same molecular weights
(36.46 a.m.u for HC1 and 38.a.m.u for F2).
3. Hydrogen Bonding:
When non-metal atoms of high electronegativity like
those of F, O and N, are linked to hydrogen, their exist
a force of attraction between positive hydrogen atom of
one molecule and negative oxygen, nitrogen or fluorine
atom of another. This force is so strong enough to cause
two or more molecules to associate in larger clusters,
as for example, (H2O) x and (NH3)
This attraction between positive hydrogen and negative
oxygen or free, is called hydrogen bond. This attraction
or "hydrogen bond" can have about 5% to 10% of the
strength of covalent bond. Hydrogen bond is
denoted by dotted lines ( ).
Hydrogen bondings differ from the word "bond* since it
is a force of attraction between positive hydrogen atom
of one molecule and negative oxygen atom of another
molecule. That is, it is an intermolecular force, not an
intra-molecular force as in the common use of word bond.
For this, hydrogen bonding is particularly strong type
of dipole-dipole inter-action.
Hydrogen bondings have an important affect on the
properties of water and ice. Hydrogen bonding is also
very important in proteins and nucleic acids and
therefore in life processes.