The hardest form of carbon is

1. Analyze the question The question asks which allotrope or form of carbon is the hardest. Carbon exists in several forms, and their hardness depends on how the atoms are bonded.

2. Explain the concept Carbon has crystalline allotropes (diamond, graphite) and amorphous forms (charcoal, coke, lampblack).

• Diamond: Each carbon atom is covalently bonded to four others in a rigid, three-dimensional tetrahedral network. This strong, uniform bonding makes it the hardest known natural substance.
• Graphite: Carbon atoms are arranged in flat hexagonal layers. The layers are held together only by weak van der Waals forces, so they slide over each other. This makes graphite soft and slippery, which is why it is used in pencils and as a lubricant.
• Charcoal and coke: These are amorphous (non-crystalline) forms with no ordered structure. They are brittle and soft, and are used as fuels and as reducing agents.

3. Solution

• A. Charcoal: amorphous and soft, so not the hardest.
• B. Coke: amorphous and brittle, so not the hardest.
• C. Diamond: has a giant tetrahedral covalent structure and is the hardest.
• D. Graphite: has a layered structure and is soft.

4. Answer The hardest form of carbon is diamond, which is option C.

5. Exam tip Remember: "Diamond = 3D, Graphite = layers." Strong bonds in all three dimensions give hardness, while layers that slide give softness. A related point that often appears in exams is that graphite conducts electricity because of its free (delocalized) electrons, while diamond does not.