Inter-Molecular Forces (Vander Waals's Forces)

There are two types of forces present in molecules, that is:

(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 bonds.

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) Hydrogen bonding.

Generally, inter molecular forces are much weaker than intra-molecular forces.

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 dispersion forces

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.

 

 
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