Van der Waals forces form electrostatic bonds between molecules. Intermolecular bonds including Van der Waals bonds hold the molecules together in liquids and solids and are responsible for phenomena such as the surface tension in liquids and crystals in solids. The intermolecular forces are much weaker than the internal forces that hold atoms together in molecules, but they are still strong enough to affect the behavior and properties of many materials.
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Electrostatic Van de Waals forces act between molecules to form weak bonds. The types of Van der Waals forces, strongest to weakest, are dipole-dipole forces, dipole-induced dipole forces and the London dispersion forces. The hydrogen bond is based on a type of dipole-dipole force that is especially powerful. These forces help determine the physical characteristics of materials.
Types of Van der Waals Forces
Three types of Van der Waals forces, strongest to weakest, are dipole-dipole forces, dipole-induced dipole forces and the London dispersion forces. Dipoles are polar molecules with negatively and positively charged poles at opposite ends of the molecule. The negative pole of one molecule attracts the positive pole of another molecule, forming an electrostatic dipole-dipole bond.
When a charged dipole molecule comes close to a neutral molecule, it induces an opposite charge in the neutral molecule, and the opposite charges attract to form a dipole-induced dipole bond. When two neutral molecules become temporary dipoles because their electrons happen to collect on one side of the molecule, the neutral molecules are attracted with electrostatic forces called the London dispersion forces, and they can form a corresponding bond.
London dispersion forces are weak in small molecules, but they increase in strength in larger molecules where many of the electrons are comparatively far away from the positively charged nucleus and are free to move around. As a result, they may collect in an asymmetrical way around the molecule, creating the temporary dipole effect. For large molecules, the London dispersion forces become a significant factor in their behavior.
When a dipole molecule contains a hydrogen atom, it can form an especially strong dipole-dipole bond, because the hydrogen atom is small and the positive charge is concentrated. The increased strength of the bond makes this a special case called the hydrogen bond.
How Van der Waals Forces Affect Materials
In gases at room temperature, molecules are too far apart and have too much energy to be affected by intermolecular Van der Waals forces. These forces become important for liquids and solids because the molecules have less energy and are closer together. The Van der Waals forces are among the intermolecular forces holding liquids and solids together and giving them their characteristic properties.
In liquids, intermolecular forces are still too weak to hold the molecules in place. The molecules have enough energy to repeatedly make and break the intermolecular bonds, sliding past one another and taking the form of their container. For example, in water, the bipole molecules are made up of a negatively charged oxygen atom and two positively charged hydrogen atoms. The water dipoles form strong hydrogen bonds holding the water molecules together. As a result, water has a high surface tension, a high heat of vaporization, and a comparatively high boiling point for the weight of the molecule.
In solids, the atoms have too little energy to break the bonds of the intermolecular forces, and they are held together with little motion. In addition to Van der Waals forces, the behavior of the molecules of solids may be influenced by other intermolecular forces, such as those forming ionic or metallic bonds. The forces hold the molecules of solids in crystal lattices such as diamonds, in metals such as copper, in homogeneous solids such as glass or in flexible solids such as plastics. While the strong chemical bonds that hold atoms together in molecules determine the chemical characteristics of materials, the intermolecular forces including the Van der Waals forces influence the physical characteristics.