Question

Which of the following is the hardest substance
A) Graphite
B) Carborundum
C) Diamond
D) Coke

Answer 1

Hint: Carbon exists in two different allotropic forms. One is crystalline form i.e. diamond and graphite and other one is amorphous form which is coal, coke, charcoal etc. And carborundum is the name used for silicon carbide. It is a semiconductor used in light emitting diodes and detectors.

Complete answer:
The crystal structure of graphite consists of hexagonal layers (sheets) of carbon atoms. These layers are linked with each other by carbon atoms through covalent bonds. The hybridisation of carbon atom is $s{p^2}$ and one electron of carbon atom is thus left unused in the 2p orbital of each carbon atom. The singly filled 2p orbital on one carbon atom overlaps with the singly filled 2p orbitals on the other C atom and forms $p\pi - p\pi $ bond. Thus, many $p\pi - p\pi$ bonds are formed. This results in an extended delocalised pi-bonding system encompassing the entire layer. Thus, graphite has a two-dimensional sheet-like structure. These layers are linked together by weak Vander Waals forces and hence can slide over there. But in the case of diamond each carbon atom is covalently bonded to four additional carbon atoms in a rigid dimensional network solid. This makes diamonds very hard.
So, diamond is the hardest substance.

Hence, the correct option is (C).

Note: Carborundum also exists in three forms diamond, zinc blende, wurtzite. While coke is a grey and has a high carbon content. It is the same in many ways to coal and its calorific value is about very high.