Introduction to Inorganic Chemistry/Ionic and Covalent Solids - Energetics

Chapter 9: Ionic and Covalent Solids - Energetics[edit]

At ordinary pressures, the CsCl structure is adopted by only three of the alkali halides, CsCl, CsBr, and CsI. Under high pressure however, other alkali halides transform from the NaCl to the CsCl structure because of the higher Madelung constant of CsCl.

In Chapter 8, we learned all about crystal structures of ionic compounds. A good question to ask is, what makes a compound choose a particular structure? In addressing this question, we will learn about the forces that hold crystals together and the relative energies of different structures. This will in turn help us understand in a more quantitative way some of the heuristic concepts we have learned about in earlier chapters, such as hard-soft acid-base theory.

Learning goals for Chapter 9:

• Understand the geometric basis of radius ratio rules.
• Understand the chemical basis of structure maps and why they are better predictors of crystal structures than radius ratios.
• Use the Born-Mayer and Kapustinskii equations to calculate lattice energies of known and hypothetical compounds.
• Construct Born-Haber cycles using lattice energies and calculate unknown quantities in the cycles.
• Predict the stabilities of low and high oxidation states using lattice energies.
• Understand the quantum mechanical origin of the extra "resonance" stability of metals.
• Predict trends in the solubility and thermal stability of inorganic compounds using lattice energies.

9.1 Ionic radii and radius ratios[edit]

Atoms in crystals are held together by electrostatic forces, van der Waals interactions, and covalent bonding. It follows that arrangements of atoms that can maximize the strength of these attractive interactions should be most favorable and lead to the most commonly observed crystal structures.

Reference: https://en.wikibooks.org/wiki/Introduction_to_Inorganic_Chemistry/Ionic_and_Covalent_Solids_-_Energetics