VAN DER FORCES
Attractive forces between uncharged molecules are known as van der Waals forces
These forces may be divided into three groups.
- Dispersion or London forces. These are due to transient Polarization.
- Dipole Dipole attraction. These are due to permanent polarization
- Hydrogen Bonding
The negative electrons in a neutral molecules are balanced by positive charges on the nucleus. Since he electrons are in motion the centre of density of electrons does not coinside continously with centre of density of positively charged nuclei, the molecules acquire an electric dipole. Polarized molecules exert an attraction for other molecules having dipole. Consider the case of Helium.
- The greater the polarizability of a molecule, the stronger are London forces
- The polarizability increases with number of electrons and their distance from nuclus
- These forces are very short range
- The inert gases have weak VAN DER WAALS forces of attraction
DIPOLE DIPOLE ATTRACTION
IT IS THE ATTRACTION BETWEEN THE POSITIVE END OF the one molecule and negative end of another molecule
H-Cl
This type of interaction is called dipole dipole interaction. The force dipole dipole attraction is inversely propotional to fourth power of their seperation of distance r
F=1/r^4
The dipole dipole attractions in gaseous molecules are particularly small since the gas molecules are in continous motion. This attraction increases the m.pt and b.pt of substances and makes their liquification easy.
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