Why is graphite soft but diamond hard?
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Answer: Different atomic arrangements
Diamond compressed under pressure — Wrong. Diamonds form under high pressure and temperature deep in Earth, but that's how they form, not why they're hard. Hardness comes from atomic structure—diamond has carbon atoms in strong 3D tetrahedral bonds, while graphite has weak layers that slide easily.
Graphite contains impurities — Wrong. Both are pure carbon. The difference is atomic arrangement: diamond has carbon atoms bonded in strong 3D structure (each atom bonded to 4 others), while graphite has layered sheets with weak bonds between layers. Structure, not purity, determines hardness.
Different atomic arrangements ✓ — Correct! Both are pure carbon, but atoms are arranged differently. Diamond: each carbon atom bonds to 4 neighbors in a rigid 3D tetrahedral structure—extremely strong in all directions. Graphite: carbon atoms form strong hexagonal sheets, but sheets are held together by weak bonds—layers slide easily, making graphite soft and slippery (used in pencils and lubricants).