Assertion–Reason on hardness of diamond and carborundum (silicon carbide) Assertion (A): Diamond and carborundum (silicon carbide) are both very hard materials. Reason (R): The valence band can be extremely strong.

Introduction / Context:
Hardness in covalent solids is governed by chemical bonding geometry and bond strength, not by abstract band-structure labels alone. Diamond and silicon carbide are classic superhard materials due to strong directional covalent bonds in robust 3D networks.

Given Data / Assumptions:

• Materials: diamond (sp^3-bonded carbon) and carborundum (SiC).
• Room-temperature mechanical hardness considered.
• Ideal, defect-minimized crystals.

Concept / Approach:
Hardness correlates with strong covalent bonding and dense, rigid crystal frameworks. In diamond, each carbon forms four short, strong sp^3 bonds in a tetrahedral network. In SiC, strong Si–C covalent bonds form a similarly stiff lattice. The phrasing “valence band can be extremely strong” is incorrect; bands describe electronic energy levels, not a mechanical entity. The correct explanation involves strong valence bonds, not “valence band.”

Step-by-Step Solution:
Assess Assertion: diamond and SiC are indeed very hard → true.Assess Reason: “valence band is strong” is a category error; mechanical hardness stems from strong covalent bonds and bonding topology.Conclusion: A true; R false.

Verification / Alternative check:
Experimental hardness scales (Vickers/Knoop) rank diamond at the top; SiC is also classified as superhard/ultrahard. Ab initio calculations tie stiffness to covalent bond strength and bulk/shear moduli, not band strength.

Why Other Options Are Wrong:
(a) suggests R correctly explains A, but it does not. (b) keeps both true, which is wrong since R is false. (d) negates A, contradicting data. (e) is incorrect because A is true.

Common Pitfalls:

• Confusing “band” (electronic energy concept) with “bond” (chemical linkage).
• Ignoring the role of 3D network connectivity and short bond length in hardness.

Final Answer:
A is true but R is false