Introduction

This chapter and the three that follow describe the cohesive forces that stabilize crystals. Each chapter concentrates on one of four limiting cases.While the limiting cases have the advantage of being easy to describe, it is important to keep in mind that real chemical bonds rarely fit exactly into one of these categories.

Our discussion of cohesive forces begins in this chapter with a description of the van der Waals bond. A brief description of dipolar bonding and hydrogen bonding is found at the end of this chapter. All three of these cohesive forces are considered to be weak and are known as secondary bonds. In comparison, the stronger ionic, metallic, and covalent bonds are considered to be primary bonds. The key assumption in the models describing secondary bonding is that the electronic energy levels of the bonded atoms are insignificantly perturbed. In other words, bonded atoms are very nearly indistinguishable from free atoms.

Each of the four chapters on bonding has been developed with a similar structure. First, the subject will be described phenomenologically, so that an intuitive understanding is developed. Second, a physical model is introduced and used, when possible, to predict measurable quantities.

Substances held together by van der Waals bonds

Van der Waals bonding plays a significant role in the cohesion of three types of solids. The first are solids containing uncharged atoms or molecular species without polar bonds, including inert gases such as He, Ne, Ar, Xe, and Kr.