Which of the following is a correct order of basicity?

Correct — D. Basic strength of the alkali metal hydroxides increases down Group 1, so the order runs CsOH, then KOH, then NaOH, then LiOH.

Going down the group the metal atom is larger and more electropositive, so the metal-to-hydroxide bond becomes more ionic.

A more ionic hydroxide releases OH⁻ into water more readily, and free OH⁻ is what makes a solution basic.

Caesium sits lowest of the four named, so CsOH is the strongest base and LiOH the weakest.

Why the others are wrong

• (a)
  LiOH>NaOH>KOH>CsOH — This is the sequence exactly reversed. It would hold only if basicity fell down the group, but electropositive character rises, so LiOH is the weakest of the four rather than the strongest.
• (b)
  KOH>CsOH>NaOH>LiOH — KOH is placed above CsOH, which breaks the trend, since caesium lies below potassium. The rest of the chain is in order, which is what makes this the tempting wrong answer.
• (c)
  LiOH>KOH>CsOH>NaOH — The order is scrambled at both ends. Lithium is the least electropositive of the four and sodium is not the weakest base, so neither the first nor the last term survives.

Group 1 holds the alkali metals - lithium, sodium, potassium, rubidium, caesium and francium.

Down the group the atom grows, the outermost electron is held more loosely, and the element turns more electropositive.

That one trend explains a family of results. The hydroxides grow more basic, the metals react more violently with water, and ionisation energy falls.

Lithium is the standing exception in Group 1 because of its small size and high charge density, and its hydroxide being the weakest base of the family fits that pattern.

Key facts

• Basic strength of alkali metal hydroxides rises down Group 1, from LiOH up to CsOH.
• The cause is rising electropositive character, which makes the metal-hydroxide bond more ionic.
• Group 1 is the alkali metals and Group 2 the alkaline earth metals.
• LiOH is both the least soluble and the least basic hydroxide of Group 1.

Common traps

• Confusing basicity as strength of a base with basicity as the count of replaceable OH groups.
• Applying the halogen reflex, where reactivity falls down the group instead of rising.

No directly related past PYQ was found.