Question:
Which of the following is not the correct statement about group 1 elements?
Which of the following is not the correct statement about group 1 elements?
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For alkali metals, melting point and boiling point decrease down the group due to weaker metallic bonding.
Updated On: Jun 15, 2026
- Hydration enthalpy is highest for \(Li^+\) ion
- Boiling point is highest for caesium
- Density of potassium is lesser than sodium and rubidium
- They have inert gas configuration in \((n-1)\) shell
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The Correct Option is B
Solution and Explanation
Concept:
Group 1 elements are alkali metals:
\[
Li,Na,K,Rb,Cs,Fr
\]
They show regular periodic trends.
Important trends:
• Atomic size increases down group
• Hydration enthalpy decreases down group
• Density generally increases except anomaly of potassium
• Melting and boiling points decrease down group
Step 1: Check option A. Smallest ion is lithium ion. Smaller ionic radius means strongest attraction with water molecules. Thus hydration enthalpy maximum for \[ Li^+ \] Hence true.
Step 2: Check option B carefully. As atomic size increases down group, metallic bond strength decreases. Therefore boiling point decreases downward. Approximate order: \[ Li& gt;Na& gt;K& gt;Rb& gt;Cs \] So caesium does NOT have highest boiling point. Hence this statement is incorrect.
Step 3: Check option C. Potassium has unusual low density. Density order approximately shows anomaly. Potassium density is lower than sodium and rubidium. Thus correct statement.
Step 4: Check option D. Electronic configuration of alkali metals: \[ ns^1 \] Inner shell has noble gas configuration. Thus statement correct. Hence incorrect statement is \[ \boxed{B} \]
• Atomic size increases down group
• Hydration enthalpy decreases down group
• Density generally increases except anomaly of potassium
• Melting and boiling points decrease down group
Step 1: Check option A. Smallest ion is lithium ion. Smaller ionic radius means strongest attraction with water molecules. Thus hydration enthalpy maximum for \[ Li^+ \] Hence true.
Step 2: Check option B carefully. As atomic size increases down group, metallic bond strength decreases. Therefore boiling point decreases downward. Approximate order: \[ Li& gt;Na& gt;K& gt;Rb& gt;Cs \] So caesium does NOT have highest boiling point. Hence this statement is incorrect.
Step 3: Check option C. Potassium has unusual low density. Density order approximately shows anomaly. Potassium density is lower than sodium and rubidium. Thus correct statement.
Step 4: Check option D. Electronic configuration of alkali metals: \[ ns^1 \] Inner shell has noble gas configuration. Thus statement correct. Hence incorrect statement is \[ \boxed{B} \]
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