15.
Explanation
Potassium (K) is the alkali metal that can form peroxides, such as potassium peroxide (K2O2). The formation of peroxides in alkali metals primarily depends on their size and the nature of their electronic configuration.
1. Size of the Alkali Metal Ion: The ability to form peroxides is related to the size of the alkali metal cation. As you move down the alkali metal group (from lithium to cesium), the size of the metal ions increases. Potassium is larger than lithium and sodium but smaller than rubidium and cesium.
2. Stability of Peroxides: Peroxides are compounds that contain the O2^2- ion, which is a peroxide ion. To accommodate this relatively large peroxide ion, the metal cation should be of a certain size. Potassium, with its intermediate size among the alkali metals, is better suited to form peroxides than smaller alkali metals like lithium and sodium.
3. Size-Dependent Ionic Radii: The ionic radii of alkali metals play a significant role. The size of the peroxide ion is comparable to the ionic radius of potassium, making it feasible for potassium to form stable peroxides.
In contrast, smaller alkali metals like lithium and sodium have cations that are too small to effectively accommodate the larger peroxide ion. Larger alkali metals like rubidium and cesium can form peroxides, but they are more likely to form superoxides (which have smaller O2^- ions) due to their larger ionic
radii.
In summary, potassium, with its intermediate size among the alkali metals, can effectively form peroxides due to the compatibility of its ionic radius with the size of the peroxide ion. This results in stable peroxide compounds like potassium peroxide (K2O2).