Which is the hardest substance?

The correct answer is Diamond.

CONCEPT:

• Diamond is a three-dimensional polymer in which a large number of carbon atoms are tetrahedrally arranged by utilizing their sp3 hybrid orbitals.
• In this structure, directional covalent bonds are present and it is very difficult to break these covalent bonds. Therefore diamond becomes the hardest substance.

EXPLANATION:

• ​Diamond is the hardest substance.
• Graphite is softer than diamond, even though both are allotropes of carbon.
• In the structure of graphite, the delocalized electron is present between the layers, which helps these layers can slide over each other.
• Therefore graphite becomes softer than diamond.