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Ch.12 - Solids and Modern Material
Chapter 12, Problem 78

Copper iodide crystallizes in the zinc blende structure. The sep- aration between nearest neighbor cations and anions is approximately 311 pm, and the melting point is 606 °C. Potassium chloride, by contrast, crystallizes in the rock salt structure. Even though the separation between nearest-neighbor cations and anions is greater (319 pm), the melting point of potassium chlo- ride is higher (776 °C). Explain.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Ionic Bonding and Lattice Energy

Ionic bonding occurs between cations and anions, where the electrostatic attraction between oppositely charged ions leads to the formation of a crystalline lattice. Lattice energy, which is the energy released when gaseous ions form a solid ionic compound, is a key factor influencing the stability and melting point of ionic compounds. Higher lattice energy typically results in a higher melting point, as more energy is required to break the ionic bonds.
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Crystal Structure Types

The arrangement of ions in a crystal lattice significantly affects the properties of ionic compounds. The zinc blende structure, adopted by copper iodide, features a tetrahedral coordination of ions, while the rock salt structure of potassium chloride has octahedral coordination. These structural differences influence the distances between ions and the overall stability of the lattice, impacting melting points.
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Influence of Ionic Size and Charge

The size and charge of the ions involved in ionic compounds play a crucial role in determining their physical properties. In potassium chloride, the larger ionic radius of potassium compared to copper, along with the higher charge density of chloride, contributes to stronger ionic interactions. This can lead to a higher melting point despite the greater separation between nearest neighbors, as the overall electrostatic interactions in the lattice are stronger.
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