Which hydroxide is least ionic in nature and tends to decompose when heated?

Alkali metal hydroxides are ionic compounds consisting of metal cations and hydroxide anions. Their ionic character depends on the size of the cation and the charge density. Lithium hydroxide (LiOH) has the smallest cation among the alkali metals. A small cation has a high polarizing power, which distorts the electron cloud of the hydroxide ion. This reduces the purely ionic character of LiOH compared to larger cations such as Na⁺, K⁺, or Cs⁺, which are less polarizing and form more purely ionic bonds.

Because of this covalent character in LiOH, it is less stable thermally and can decompose upon heating to release lithium oxide (Li₂O) and water. The reaction is represented as:

LiOH → Li₂O + H₂O

In contrast, NaOH, KOH, and CsOH are more ionic in nature due to their larger cation sizes, which have lower polarizing power. These hydroxides are highly stable at high temperatures and do not easily decompose on heating.

The trend in ionic character of alkali metal hydroxides is:

CsOH > KOH > NaOH > LiOH

This trend also explains why LiOH behaves differently from the rest. Understanding this property is important in inorganic chemistry, especially when studying alkali metal chemistry, thermal stability of compounds, and bonding trends.

This MCQ highlights the relationship between cation size, ionic versus covalent character, and thermal stability. Such knowledge is essential for students to predict the chemical behavior of hydroxides and to understand why LiOH is unique among the alkali metal hydroxides.