Question

The hardest known substance is ___________ .
A. Graphite
B. Diamond
C. Tungsten
D. Fullerene

Answer 1

Hint: It is an allotrope of carbon. And also, it is used as industrial abrasives for cutting hard materials. It has a high index of refraction, high dispersion, and high lustre. It is the world's most popular gemstone.

Complete step by step answer:

Both option A and option B are the allotropes of carbon. As we know that these minerals chemically consist of carbon atoms with different physical properties. We should know that these allotropes of carbon, the atoms consisting of carbon atoms in that of the Diamond and Graphite, are bound together by strong covalent bonds with different arrangements.
But due to one difference diamond became the hardest known substance. We know that diamonds are said to have strong chemical bonds with that of the four other carbon atoms, thus making a perfect tetrahedron structure and on throughout the crystal. The carbon atoms here are \[s{{p}^{3}}\] hybridized, and the bond lengths of carbon-carbon atoms are equal. Hence, diamonds form a three - dimensional network of strong covalent bonds.
Whereas, graphite forms sheet like structure. The carbon atoms in the Graphite structure are \[s{{p}^{2}}\] hybridized and are thus forming hexagonal rings. Due to this we find out that layers slide over one another and that’s why graphite is smooth and used as lubrication.
Tungsten is hard but not harder than diamond. On Mohs scale of hardness we find out that tungsten measures from 8 or 9, but diamond measures to 10.
As we know that fullerene is an allotrope of carbon whose molecule consists of carbon atoms connected by single and double bonds so as to form a closed or partially closed mesh, with fused rings of five to seven atoms. A fullerene is a little bit like a small sheet of graphite that has been curved into a ball. But fullerene is not harder than diamond.
So, from the above points, now we can conclude that diamond is the hardest known substance.
So, option B is our correct answer.

Note: Mohs scale is the rough measure of the resistance of a smooth surface to scratching or abrasion. We should know that the Mohs hardness of a mineral is determined by observing whether its surface is scratched by a substance of known or defined hardness.

Increasing scale of hardness on Mohs scale:
\[\begin{align}
  & Talc\to Gypsum\to ~Calcite\to ~Fluorite~\to ~Apatite\to ~Orthoclase \\
 & \to ~Quartz\to ~Topaz~\to ~Corundum\to Diamond \\
\end{align}\]