Before you go to discuss about what is London dispersion forces.
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Now, we going to discuss about all information of London dispersion forces, example, and small intesresting history also.
What is London dispersion forces?
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# London Dispersion Force: The study of intermolecular forces present among nonpolar molecules or the individual atoms of a noble gas is very interesting.
For example, Benzene (C6 H6) has zero dipole moment and experiences no dipole-dipole forces, yet exists in liquid stage.
In case of nonpolar molecules and inert gases, only dipersion forces exist. Dispersion forces are also called as London forces due to an idea of momentary dipole which was proposed by the German Physicist, Fritz London in 1930. These forces are also called as van der Waals forces.
It is the weakest intermolecular force that develops due to interaction between two nonpolar molecules. In general, all atoms and molecules experience London dispersion forces, which result from the motion of electrons. At any given instant of time, the electron distribution in an atom may be asymmetrical, giving the atom a short lived dipole moment. This momentary dipole on one atom can affect the electron distribution in the neighboring atoms and induce momentary dipoles in them. As a result, weak attractive force develops.
For example, substances composed of molecules such as O², CO², N²,halogens, methane gas, helium and other noble gases show van der Waals force of attraction.
The strength of London forces increases with increase in molecular size, molecular mass and number of electrons present in an atom or molecule.
When two nonpolar molecules approach each other, attractive and repulsive forces between their electrons and the nuclei will lead to distortions in the size of electron cloud, a property referred to as polarizability.
Polarizability is a measure of how easily an electron cloud of an atom is distorted by an applied electric field. It is the property of atom. The ability to form momentary dipoles that means the ability of another molecule to become polar by redistributing its electrons is known as polarizability of the atom or molecule.
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| Fig: 10.3 |
More the number of electrons in a molecule, hight is its ability to become polar. Similarly more the spread out shapes higher the dispersion force present between the molecules. London dispersion force are stronger in a long chain of atoms where molecules are not compacts. This can affect physically property such as B.P. n-pentane, for example boils at 309.4 K
, whereas neo-pentane boils at 282.7 K, Here both the substances have the molecular formula, C5 H¹², but n-pentane is longer and somewhat spread out, where as neo -pentane is more spherical and compact. As shown in fig :10.3.
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