Johannes van der Waals developed a model to explain the behavior of liquids and solids that reflected the size of the atoms or molecules and the strength of the forces between these atoms or molecules. The weak intermolecular forces in liquids and solids are called van der Waals forces. There are three categories of these forces: dipole-dipole, dipole-induced dipole, and induced dipole-induced dipole. The number of electrons, the size of the molecule, and the shape of the molecule determine the magnitude of van der Waals forces.
When the force of attraction between molecules is weak, the substance is likely a gas at room temperature. When the force of attraction between molecules is strong, the substance is likely to be a solid. Most metals are solid at room temperature. Van der Waals forces produce the orderly pattern of molecular crystals. When the kinetic energy of molecules is great enough to overcome part of the van der Waals forces, the molecules have more mobility and the substance turns into a liquid. Molecules in a liquid are almost as close together as the molecules in a solid. Small groups of molecules still stick together in a liquid, but the molecules can move over one another with ease allowing the liquid to take the shape of the container and to flow. There are just enough van der Waals forces left in a liquid to give liquids a definite volume.
Solid carbon dioxide is dry ice. The van der Waals forces holding carbon dioxide together are so weak that dry ice sublimes or goes directly from the solid to the gas phase.
It takes a trained scientific mind to realize that the properties of water are unusual when compared to other molecules. Water has strong intermolecular forces or van der Waals forces. Solids usually expand when they melt. Water expands when it freezes. Solids are usually denser than their liquids. Ice is not as dense as water so ice floats. Water has dipole-dipole van der Waals forces. Water molecules have a positive and a negative end and behave like two bar magnets.
Some molecules shift the electrons to one side which distorts the electron orbit. The intermolecular interactions between these distorted molecules produce the induced dipole-induced dipole attraction forces. Fluctuating dipoles is another way to describe two distorted molecules. Neighboring helium atoms behave in this way; all inert gases have induced dipole-induced dipole attraction forces. Dipole-induced dipole forces are found in mixtures.
