As a rule, ionic compounds have higher melting points, and therefore higher boiling points also. Melting points and boiling points of compounds depend on the strength of the intermolecular forces of attraction between the molecules, or in the case of KCl, between the positive and negative ions. The latter is quite strong, and in the solid is simply referred to as ionic bonding. Intermolecular forces between nonpolar molecules such as Cl2 are much lower, so the molecules don't "stick" to each other very strongly, making is easier to free them up enough by heating so that they move about more freely (in the liquid state after melting) or separate completely (in the gaseous state after boiling).
There are several types of intermolecular attractive forces that may be listed in this chapter that are not in the category of normal chemical bonding (ionic, covalent, or metallic bonding):
1. Ion-dipole interactions. These occur between ions like Na+ or Cl- and the polar water solvent molecules.
2. Dipole-dipole interactions. Occur between polar molecules. Water molecules themselves have strong dipole-dipole interactions between each other, because water is a very polar molecule. When a hydrogen atom is involved, as is the case with water, this dipole-dipole interaction is called "hydrogen bonding".
3. London dispersion forces (sometimes called van der Waals interactions). These are comparatively weaker than ion-dipole and dipole-dipole interactions. London dispersion interactions occur between nonpolar molecules. The larger the surface area of contact between the molecules, the greater the interaction. This is the type of interaction that occurs between nonpolar Cl2 molecules. Since both atoms in Cl2 are the same type, with the same electronegativity, the bond between them will be nonpolar.
Steve
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