Question

The most basic hydroxide among those of alkali metals is,
$(i){\text{ LiOH}}$
$(ii){\text{ NaOH}}$
$(iii){\text{ CsOH}}$
$(iv){\text{ KOH}}$

Answer 1

More easily the ionization of hydroxide takes place, the basicity of the compound thus increases. The compound whose hydroxide ion gets easily ionised will be the most basic compound among the following alkali metal hydroxides. It depends on the size of cation and anion also.

Complete Answer:
Alkali metals group is the first group of modern periodic table. These metals easily lose electrons as their size is small as compared to metal atoms of other groups of the same period. Thus these metals are mostly acidic in nature. But the hydroxides of these metals are basic in nature. The basicity of the compounds depends on the ionization of the hydroxide ions. The compound whose hydroxide gets more easily ionized will be the most basic among all. Ionization of hydroxide depends on the size of the cation. Thus the cation of cation of size must be larger so that hydroxide ion gets easily ionized. Thus we know that as we move down the group the size of metal atoms increases progressively. Therefore the size order of the alkali metals can be observed as,
$Li{\text{ }} \prec {\text{ Na }} \prec {\text{ K }} \prec {\text{ Rb }} \prec {\text{ Cs}}$ Increasing order of size of alkali metals
Thus we can observe that Cs has the largest size among the given alkali metals. Therefore the most basic hydroxide will be ${\text{ CsOH}}$.

Thus the correct option is $(iii){\text{ CsOH}}$.

Note:
As the size of cation is increased the electron density of anion also increases which will increase the basic character of the hydroxides. Thus the compound which has a large size of cation will be the most basic alkali metal hydroxides. The least basic will be ${\text{LiOH}}$.