Bonding and Structures of Ionic Compounds

When metals and nonmetals react, the resulting ionic compounds are very stable large amounts of energy are required to take them apart. For example, the melting point of sodium chloride is approximately 800 °C. 

The strong bonding in these ionic compounds results from the attractions between the oppositely charged cations and anions. 

Structures of Ionic Compounds We write the formula of an ionic compound such as lithium fluoride simply as LiF, but this is really the empirical, or simplest, formula. The actual solid contains huge and equal numbers of 

The structures of virtually all binary ionic compounds can be explained by a model that involves packing the ions as though they were hard spheres. The larger spheres (usually the anions) are packed together, and the small ions occupy the interstices (spaces or holes) among them. 

The structure of lithium fluoride, (a) This structure represents the ions as packed spheres. 

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This theory is applicable to the molecules formed by covalent bonds because these are directional. The force that holds ions together in the ionic compounds is the coulombic force. This force is non-directional and depends on the distance between the ions only. The crystal structure of ionic compound is, therefore, determined by relative sizes and charge of the ions. 

I Describe the formation of ionic bonds and the structure of ionic compounds. 

Note While the carbonate is an ion and is found in ionic compounds, the bonding between the carbon atom and the oxygen atoms is covalent and the Lewis structure is determined by the steps in Table 6.1. This is true of all polyatomic ions. The Lewis electron-dot structures of ionic compounds that contain polyatomic ions are discussed in Section 6.7. 

In Chapters 2 and 3, we considered the history, nomenclature, and structures of coordination compounds. In these earlier discussions, we introduced the metal-ligand (M-L) coordinate-covalent bond in which the ligand shares a pair of electrons with the metal atom or ion. Now we are in a position to consider the nature of the M-L bond in greater detail. Is it primarily an ionic interaction between ligand electrons and a positively charged metal cation Or should the M-L bond be more properly described as predominantly covalent in character Whatever the character of the bond, the description of M-L interactions must account for (1) the stability of transition metal complexes, (2) their electronic and magnetic characteristics, and (3) the variety of striking colors displayed by these compounds. 

With less polar solvents and more basic allyl anions the compounds are present as ion pairs. The carbon-metal bond with the alkali and alkaline earth metals are known to have high ionic character. The allyl compounds behave accordingly as salts. The structures of allyl compounds of the alkali and alkaline earth metals are of two fundamental types, a 41 (or metal cation is associated closely with a single terminal allylic carbon, and the rf 1 (or ji) type, 15, in which the cation bridges the two terminal allylic positions. 

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