Molecular Forces

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Forces:-
   There are two types of forces between molecular.
1.      Intramolecular force
2.    Intermolecular force
Intramolecular force:-
                           A force within a molecular is called intramolecular force.
I.      Ionic bond     II.    Covalent bond
Intermolecular force:-
Intermolecular forces in liquids are collectively called van
der Waals forces. These forces are essentially electrical in nature
And result from the attraction of charges of opposite sign.
The principal kinds of intermolecular attractions are:
(1) Dipole-dipole attractions
(2) London forces
(3) Hydrogen bonding.

The relative size of these interactions is important so the relative effects are understood.

 Clearly normal covalent bonds are almost 40 times the strength of hydrogen bonds. Covalent
Bonds are almost 200 times the strength of dipole-dipole forces, and more than 400 times the size of
London dispersion forces.
DIPOLE–DIPOLE ATTRACTIONS
Dipole-dipole attractions exist between molecules that are polar. This requires the presence of polar bonds and an unsymmetrical molecule. These molecules have a permanent separation of positive and negative charge.
                            In the illustration the H and of HCl is permanently slightly positive charge. The Cl end of HCl has a permanent slight negative charge. The H atom in one molecule is attracted to the Cl in a neighbour. The intermolecular force is weak compared to a covalent bond, but
This dipole-dipole interaction is one of the stronger intermolecular attractions

 LONDON DISPERSION FORCES
London dispersion forces exist in nonpolar molecules. These forces result from temporary charge imbalances. The temporary charges exist because the electrons in a molecule or ion move randomly in the structure. The nucleus of one atom attracts electrons form the neighbouring atom. At the same time, the electrons in one particle repel the electrons in the neighbour and create a short lived charge imbalance.
These temporary charges in one molecule or atom attract opposite charges in nearby molecules or atoms. A local slight positive charge δ+ in one molecule will be attracted to a temporary slight negative charge δ– in a neighbouring molecule.

HYDROGEN BONDING
Hydrogen bonding is a unique type of intermolecular attraction. There are two requirements:
(1) Covalent bond between an H atom and either F, O, or N. These are the three most electronegative elements.
(2) Interaction of the H atom in this kind of polar bond with a lone pair of electrons on a nearby atom like F, O, or N.

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