Which of the following crystals shows 4:2 coordination? (a) CaF2 (b) SiO2 (c) PbO2 (d) None of the above

To determine which of the given crystals exhibits 4:2 coordination, we first need to understand what this coordination means. In coordination chemistry, the notation "4:2" typically refers to the ratio of cations to anions in a crystal structure. This means that for every four cations, there are two anions present in the structure. Let's analyze each option to see which one fits this description.

Analyzing the Options

• CaF2 (Calcium Fluoride): In this compound, calcium ions (Ca²⁺) are surrounded by fluoride ions (F⁻). The structure is known as a face-centered cubic lattice where each calcium ion is coordinated by eight fluoride ions, giving a coordination number of 8:4. Thus, this does not match the 4:2 ratio.
• SiO2 (Silicon Dioxide): Silicon dioxide forms a tetrahedral structure where each silicon atom is surrounded by four oxygen atoms. The ratio here is 1:2 (1 silicon for every 2 oxygens), which also does not correspond to 4:2.
• PbO2 (Lead Dioxide): In lead dioxide, lead ions (Pb²⁺) are coordinated by oxygen ions (O²⁻). The structure typically shows a coordination of 4 lead ions to 2 oxygen ions, which gives a 4:2 ratio. This is the only option that fits the criteria.
• None of the above: Since we have identified PbO2 as having a 4:2 coordination, this option is not applicable.

Conclusion

Based on the analysis, the correct answer is (c) PbO2. This compound demonstrates a 4:2 coordination, where four lead ions are coordinated to two oxygen ions. Understanding the coordination numbers and how they relate to the crystal structure is crucial in materials science and chemistry, as it helps us predict the properties and behaviors of different compounds.