What is a nonstoichiometric compound? Give an example, and account for its lack of stoichiometry in terms of structure.

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Nonstoichiometric compounds are chemical compounds that do not adhere to the simple integer ratio of atoms as dictated by their chemical formula.

These compounds often arise due to defects in the crystal lattice, such as vacancies or interstitial atoms, which lead to a deviation from the ideal stoichiometric ratio.

An example of a nonstoichiometric compound is wüstite, which is often represented as FeO, but in reality, it has a formula closer to Fe_{0.95}O.

In wüstite, the nonstoichiometry is due to the presence of iron vacancies in the crystal lattice, which means there are fewer iron atoms than oxygen atoms compared to the ideal 1:1 ratio.

These vacancies can occur because of the variable oxidation states of iron, allowing for a range of compositions while maintaining charge balance in the crystal structure.

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Key Concepts

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Nonstoichiometric Compounds

Nonstoichiometric compounds are materials that do not have a fixed ratio of their constituent elements, meaning their composition can vary. This variability often arises from defects in the crystal structure, such as vacancies or interstitials, which can lead to different ratios of elements than those predicted by simple stoichiometry.

Crystal Structure

The crystal structure of a compound refers to the orderly arrangement of atoms within a crystalline solid. In nonstoichiometric compounds, the presence of defects or irregularities in this structure can disrupt the ideal ratios of elements, resulting in a composition that deviates from the expected stoichiometric formula.

Examples of Nonstoichiometric Compounds

Common examples of nonstoichiometric compounds include metal oxides like FeO and TiO2, where the ratio of metal to oxygen can vary. For instance, in iron oxide, the presence of iron vacancies can lead to a composition that is not strictly 1:1, illustrating how structural variations can affect stoichiometry.