This chapter covers covalent bonding in small molecules, starting from hydrogen molecule ions and hydrogen molecules, and then proceeding to homonuclear and heteronuclear diatomic molecules, and linear triatomics. Using the hydrogen molecule as the prototype, the bonding of this molecule is treated by the molecular orbital and valence bond theories. The difference between these two models is discussed in detail. In addition, since both methods are approximate, refinement of the approximation will lead to identical results. The hybridization theory is also discussed, and several systems that may be effectively dealt with by this theory are provided. The chapter concludes with a discussion of the Hückel molecular orbital treatment of conjugated polyenes. The wavefunctions of the polyenes are used to predict the products of the cyclization reactions of the polyenes.
Keywords: antibonding molecular orbital, bonding molecular orbital, covalent radii, diatomic molecules, Hückel molecular orbital theory, hybridization schemes, molecular orbital theory, nonbonding orbital, overlap integral, valence bond theory