Question

Assertion (A): All noble gases are monatomic.
Reason (R): All noble gases have very low melting and boiling points.

• A. Both (A) and (R) are correct and (R) is the correct explanation of (A).
• B. Both (A) and (R) are correct but (R) is not the correct explanation of (A).
• C. (A) is correct but (R) is incorrect.
• D. (A) is incorrect but (R) is correct.

Hint:

Noble gases are monatomic because they possess completely filled valence shells. Their very low melting and boiling points arise from weak London dispersion forces between the atoms.

Answer 1

The Correct Option is B

Step 1: Analyze Assertion (A).
Noble gases such as \[ He,\ Ne,\ Ar,\ Kr,\ Xe,\ Rn \] have completely filled valence shells.
Because of their stable electronic configurations, they have very little tendency to gain, lose, or share electrons.
Therefore, noble gases generally exist as single atoms rather than forming molecules.
Hence, all noble gases are monatomic.
Thus, Assertion (A) is correct.

Step 2: Analyze Reason (R).
Noble gas atoms interact only through weak London dispersion forces.
Since these intermolecular forces are very weak, only a small amount of energy is required to separate the atoms.
As a result, noble gases have very low melting points and boiling points.
Hence, Reason (R) is also correct.

Step 3: Check whether (R) explains (A).
The monatomic nature of noble gases is due to their completely filled valence shells and chemical inertness.
The low melting and boiling points are a consequence of weak intermolecular forces between already existing monatomic atoms.
Therefore, low melting and boiling points do not explain why noble gases are monatomic.
Thus, (R) is not the correct explanation of (A).

Step 4: Final conclusion.
Both Assertion (A) and Reason (R) are correct, but Reason (R) is not the correct explanation of Assertion (A).
Therefore, \[ \boxed{\text{Both (A) and (R) are correct but (R) is not the correct explanation of (A).}} \] Hence, the correct option is \[ \boxed{(2)} \]