Question

Which from following statements is NOT correct?

• A. All alkali metals are silvery white.
• B. Density of potassium is less than sodium.
• C. Compounds of group-1 elements are diamagnetic.
• D. Melting point of group-1 elements increase down the group.

Hint:

Logic Tip: Unlike halogens whose melting/boiling points increase down the group due to increasing van der Waals forces, metals typically see a decrease in melting points down a group because larger atomic radii result in weaker metallic bonds.

Answer 1

The Correct Option is D

Concept:
Let's evaluate the periodic trends and properties of Group-1 elements (alkali metals) to identify the incorrect statement.

• Alkali metals are generally silvery-white, soft, and light metals.
• Due to an anomalous increase in atomic radius, potassium ($0.86 \text{ g/cm}^3$) is exceptionally less dense than sodium ($0.97 \text{ g/cm}^3$).
• Group-1 ions form by losing their single valence electron, achieving a noble gas configuration with all paired electrons, making their compounds diamagnetic.
• Melting points depend on the strength of metallic bonding.

Step 1: Analyze the melting point trend for alkali metals.
As we move down Group 1 (from Li to Cs), the atomic size increases. Because the atoms are larger, the distance between the metal nuclei and the delocalized valence electrons increases. This leads to a \textit{decrease} in the strength of the metallic bonding.
Step 2: Conclude the validity of the statements.
Because the metallic bonds become weaker down the group, less thermal energy is required to break the lattice. Therefore, the melting point of Group-1 elements \textit{decreases} down the group, not increases. Thus, statement (D) is incorrect.