Science (086) — AI-generated practice question

In diamond, each carbon atom forms covalent bonds with four other carbon atoms in a rigid three-dimensional network. This extensive cross-linking leaves no free electrons and creates an extremely strong, compact structure with no weak points, making diamond the hardest known substance.

In graphite, each carbon atom bonds to only three other carbon atoms in flat hexagonal layers. The fourth valence electron forms a weak force between the layers. These layers can slide easily over one another, making graphite smooth and slippery.

Thus, the difference in bonding pattern — 4 bonds (3D) vs. 3 bonds (layered) — explains their contrasting physical properties.

Source: Chapter 4, Allotropes of Carbon (More to Know!)

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• Examiners look for: (1) diamond — 4 bonds, 3D rigid network → hardness; (2) graphite — 3 bonds, layered planes → slipperiness. Award 1 mark each, with 1 mark for linking bond type to property.
• Do not say "ionic bonds" — these are covalent structures.
• Mentioning the "fourth valence electron / double bond" in graphite as the inter-layer weak force is a bonus point but keep it brief.
• Avoid over-writing; the passage itself is your authority here.