what are the relative strengths of the different intermolecular forces?
http://en.wikipedia.org/wiki/Intermolecular_force
The relative strengths of different intermolecular forces can be determined by considering their molecular properties and the nature of interactions between molecules. Here are some of the main types of intermolecular forces and their relative strengths:
1. London dispersion forces (also known as Van der Waals forces): These are the weakest intermolecular forces and occur in all molecules. They result from temporary fluctuations in electron distribution, creating temporary dipoles. The strength of London dispersion forces increases with increasing molecular size and shape, as larger and more symmetrical molecules have a greater number of electrons available for temporary dipoles.
2. Dipole-dipole interactions: These forces occur between polar molecules and are stronger than London dispersion forces. In dipole-dipole interactions, the positive end of one molecule attracts the negative end of another molecule. This occurs because polar molecules have a permanent dipole moment due to differences in electronegativity between atoms. The strength of dipole-dipole interactions increases with increasing polarity of molecules.
3. Hydrogen bonding: This is a special type of dipole-dipole interaction that occurs between a hydrogen atom bonded to a highly electronegative atom (such as oxygen, nitrogen, or fluorine) and a lone pair of electrons on another electronegative atom. Hydrogen bonding is stronger than regular dipole-dipole interactions due to the high polarity of the hydrogen atom. It is an important force in substances like water and ammonia.
It is important to note that these forces exist on a continuum, with London dispersion forces being the weakest and hydrogen bonding being the strongest. The strength of intermolecular forces can influence various properties of substances, such as boiling points, melting points, and solubilities.