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.

Verified step by step guidance

1. insert step 1> Understand the structures: Copper iodide (CuI) crystallizes in the zinc blende structure, which is a type of face-centered cubic lattice. Potassium chloride (KCl) crystallizes in the rock salt structure, which is also a face-centered cubic lattice but with a different arrangement of ions.
2. insert step 2> Consider the ionic interactions: In both structures, the melting point is influenced by the strength of the ionic bonds between the cations and anions. The strength of these interactions depends on the charge and size of the ions.
3. insert step 3> Compare ionic sizes and charges: Copper (Cu) and iodide (I) ions in CuI have different sizes and charges compared to potassium (K) and chloride (Cl) ions in KCl. The charge on Cu is +1 and on I is -1, similar to K and Cl, but the size and polarizability of the ions differ.
4. insert step 4> Analyze the effect of ionic size and polarizability: Although the separation between ions in KCl is greater, the larger size and higher polarizability of iodide ions in CuI can lead to weaker ionic interactions compared to the smaller and less polarizable chloride ions in KCl.
5. insert step 5> Conclude with melting point implications: The stronger ionic interactions in KCl, due to the smaller and less polarizable chloride ions, result in a higher melting point compared to CuI, despite the greater separation between ions in KCl.

Key Concepts

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.

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.

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.