Rubidium iodide crystallizes with a cubic unit cell that contains iodide ions at the corners and a rubidium ion in the center. What is the formula of the compound?

Ionic compounds are a class of chemical compounds comprised of positively charged ions called cations and negatively charged ions known as anions. These oppositely charged ions attract each other due to electrostatic forces and form a stable ionic bond. The most common types of cations are metals, while anions are typically non-metals or polyatomic ions.

For example, in the case of rubidium iodide (RbI), rubidium (Rb) acts as the cation, and iodide (I−) is the anion. Rubidium loses an electron to achieve a stable electron configuration, becoming Rb+, while iodide gains an electron, becoming I−. Together, they form RbI through ionic bonding. It is crucial for students to understand that the formula of any ionic compound reflects the ratio of cations to anions needed to maintain electrical neutrality in the compound.

The cubic unit cell is the simplest type of crystal lattice structure and serves as a building block for more complex structures. A unit cell is the smallest repeating unit that shows the full symmetry of the crystal and can be used to construct the entire crystal by stacking it in three-dimensional space.

In a cubic unit cell, all edges are equal in length, and each corner of the cell typically holds an ion, which can be shared by adjacent unit cells. Understanding how these ions are arranged and how they contribute to the entire crystal structure is integral when determining the stoichiometry and physical properties of the crystalline material.

Stoichiometry in a crystal lattice involves understanding the quantitative relationships between the ions in the crystal structure. It dictates the simple ratio of the different ions that form a compound and can be determined by examining the unit cell. Each ion's position and the degree to which it is shared with neighboring cells is crucial when calculating the stoichiometry.

For instance, in a rubidium iodide crystal, a cubic unit cell configuration, an iodide ion (I−) is present at each corner and a rubidium ion (Rb+) at the center. Since the corners are shared among eight unit cells, each corner ion contributes 1/8th to the unit cell. With eight corners, this leads to a total contribution of one I− per unit cell. The Rb+ ion in the center contributes entirely to one cell, leading to a clear 1:1 stoichiometric ratio of Rb+ to I− ions, thus forming the compound RbI.