Assertion–Reason (solid state basics): Assertion (A) The atomic number of sodium is 11. Reason (R) Sodium crystallizes in a body-centred cubic (BCC) lattice. Evaluate the pair.

Introduction / Context:
Assertion–Reason questions test whether a correct statement (the assertion) is logically explained by the accompanying reason. Here, we compare an elemental property (atomic number) with a crystal structure property (lattice type) for sodium (Na).

Given Data / Assumptions:

• Atomic number Z(Na) = 11, corresponding to electron configuration [Ne] 3s^1.
• At ambient conditions, metallic sodium adopts the BCC crystal structure.

Concept / Approach:

Atomic number is a nuclear property counting protons; it is independent of the solid’s crystal lattice. While valence electron configuration influences bonding and thus often correlates with preferred lattice, the numeric fact “Z = 11” does not follow from, nor is it explained by, “Na is BCC.” Therefore both statements are true, but the reason does not explain the assertion.

Step-by-Step Solution:

Verification / Alternative check:

Chemical data tables list Z = 11 and BCC structure for alkali metals (Li, Na, K, etc.) at standard conditions, supporting both independent facts.

Why Other Options Are Wrong:

• A true, R false: incorrect because Na is indeed BCC at room temperature.
• A false, R true or both false: contradict established facts.
• “R explains A” is logically incorrect; lattice does not determine atomic number.

Common Pitfalls:

• Assuming correlation implies causation; lattice type does not determine Z.

Final Answer:

Both A and R are true but R is not correct explanation of A