You may be wondering, why should neutral molecules attract each other at all? If the molecules are polarDescribes a molecule that has separated, equal positive and negative charges that consitute a positive and a negative pole; such a molecule tends to assume certain orientations more than others in an electric field., the explanation is fairly obvious. When two polar molecules approach each other, they can arrange themselves in such a way that the negative side of one molecule is close to the positive end of the other:
The molecules will then attract each other because the charges which are closest together are opposite in sign. (This behavior is very similar to the attraction between two bar magnets placed end to end or side by side with the north poles opposite the south poles.) Forces between polar molecules which arise in this way are called dipole forces. The existence of dipoleIn an electrically neutral species, separated, equal positive and negative charges that consitute a positive and a negative pole; such a species tends to assume certain orientations more than others in an electric field. forces explains why polar molecules have higher boiling points and melting points than do nonpolarDescribes a molecule with no net permanent dipole; this can occur when there is no separation of centers of positive and negative electrical charge or because there are bond dipoles that cancel each others' effects. A polar molecule will assume certain orientations more than others in an electric field. molecules. In the following table, we compare the boilingThe process of a liquid becoming vapor in which bubbles of vapor form beneath the surface of the liquid; at the boiling temperature the vapor pressure of the liquid equals the pressure of the gas in contact with the liquid. points of several pairs of molecules. In each pair, one molecule is polar and the other is nonpolar, but otherwise they are as similar as possible. The polar substanceA material that is either an element or that has a fixed ratio of elements in its chemical formula. always has the higher boiling point, indicating greater attractive forces between separate molecules, that is, larger intermolecular forces.
Boiling Points of Otherwise Similar Polar and Nonpolar Substances.
|Nonpolar Molecules||Polar Molecules|
|Molecule||Molar Mass / g mol-1||Total Number of Electrons||Boiling Point (in degrees C)||Molecule||Molar Mass / g mol-1||Total Number of Electrons||Boiling Point (in degrees C)|