Question

Which of the following is not the correct statement about group 1 elements?

• A. Hydration enthalpy is highest for \(Li^+\) ion
• B. Boiling point is highest for caesium
• C. Density of potassium is lesser than sodium and rubidium
• D. They have inert gas configuration in \((n-1)\) shell

Hint:

For alkali metals, melting point and boiling point decrease down the group due to weaker metallic bonding.

Answer 1

The Correct Option is B

Step 1: Recall the alkali metal trends.
Group 1 metals are Li, Na, K, Rb, Cs, Fr, and we must find the statement that is wrong.
Step 2: Check the hydration enthalpy claim.
The smallest ion $\text{Li}^+$ binds water most strongly, so its hydration enthalpy is the highest. That statement is correct.
Step 3: Check the boiling point claim.
Going down the group atoms grow larger and metallic bonding weakens, so boiling points fall in the order Li > Na > K > Rb > Cs. Caesium therefore has the lowest, not the highest, boiling point, so this statement is wrong.
Step 4: Check the density claim.
Potassium shows a density anomaly and is indeed lighter than both sodium and rubidium, so that statement is correct.
Step 5: Check the configuration claim.
Each alkali metal has an $ns^1$ outer electron over a noble gas core in the $(n-1)$ shell, so that statement is correct.
Step 6: Pick the incorrect one.
Only the boiling point statement is false, which is option 2.
\[ \boxed{\text{Boiling point is highest for caesium}} \]