Basic Concepts of Chemical Bonding
On the table in most diners you can expect to find two white, crystalline substances: table salt and granulated sugar. In spite of their similarities in appearance, salt and sugar are vastly different kinds of substances. Table salt is sodium chloride, NaCl, which
consists of sodium ions, Na+ and chloride ions, Cl-. The structure is held together by the attractions between the oppositely charged ions, which we call ionic bonds. Granulated sugar, in contrast, does not contain ions at all. It consists of molecules of sucrose, C12H22O11, in which attractions called covalent bonds hold the atoms together. One consequence of the difference in bonding in salt and sugar is their different behaviors in water: NaCl dissolves in water to yield ions in solution (NaCl is an electrolyte), whereas sucrose dissolves in water to yield aqueous C12H22O11 molecules (sucrose is a nonelectrolyte). (Section 4.2)
The properties of substances are determined in large part by the chemical bonds that hold their atoms together. What determines the type of bonding in each substance, and just how do the characteristics of these bonds give rise to different physical and chemical properties? The keys to answering the first question are found in the electronic structures of the atoms involved, which we discussed in Chapters 6 and 7. In this chapter and the next we will examine the relationships among electronic structure, chemical bonding forces, and chemical bond type. Well also see how the properties of ionic and covalent substances arise from the distributions of electronic charge within atoms, ions, and molecules.