Which one of the following hydroxides is least ionic in nature and decomposes on heating?

Among alkali metal hydroxides, the ionic character depends on the size of the metal cation. Lithium (Li⁺) is the smallest cation in Group 1, with a high charge density. Its strong polarizing power distorts the electron cloud of the hydroxide ion (OH⁻), which reduces the purely ionic nature of LiOH. In contrast, larger alkali cations like Na⁺, K⁺, and Cs⁺ produce hydroxides with stronger ionic bonds because their polarizing power is lower.

Lithium hydroxide also exhibits thermal decomposition due to the covalent character introduced by the small Li⁺ ion. When heated, LiOH decomposes into lithium oxide (Li₂O) and water:

2LiOH→ΔLi2O+H2O2LiOH \xrightarrow{\Delta} Li₂O + H₂O2LiOHΔLi2O+H2O

Other alkali metal hydroxides, such as NaOH, KOH, and CsOH, are more stable on heating and do not decompose easily because their larger cations reduce polarization of OH⁻, making their bonds more ionic and thermally stable.

This behavior of LiOH is closely related to Fajan’s rules, which state that small, highly charged cations tend to polarize the anion, increasing covalent character and decreasing ionic character. The high polarizing power of Li⁺ makes LiOH less ionic compared to the hydroxides of heavier alkali metals.

Understanding the properties of LiOH is important in inorganic chemistry, as it explains why lithium behaves differently from other Group 1 elements in reactions, thermal stability, and solubility. It also highlights the unique characteristics of lithium compounds due to the small size of Li⁺.

Hence, among the given options, lithium hydroxide (LiOH) is the least ionic and decomposes on heating, distinguishing it from other alkali metal hydroxides.