Why is diamond hard and graphite brittle while both are made up of carbon atoms?
A. Difference in the melting point
B. Difference in the conductivity
C. Difference in the nature of carbon bonds
D. Difference in the boiling point

Hint: Diamond and graphite are the allotropes of the carbon. They have different physical forms and the properties. This difference in the physical form is due to the type of bonds formed between the carbon atoms in the formation of the respective compound.

Complete Solution :
Besides having the different physical properties there is the structural difference between the diamond and graphite that is
- In diamond there are 4 sigma bonds that are formed with the each adjacent carbon atom and the carbon atom in the diamond undergoes $s{{p}^{3}}$ hybridization which makes the diamond strong
- Whereas in case of graphite there are 3 sigma bonds that are formed with the each adjacent carbon atom and the carbon atom in the graphite undergoes $s{{p}^{2}}$ hybridization also the layers of graphite are held by weak Van der Waals forces which make it soft.
- Due to the nature of bonds formed between the carbon atoms in the structure of diamond and graphite are different, the hardness of the diamond and graphite are also different.
So, the correct answer is “Option C”.

Note: The diamond and graphite are also known as the allotropes of the carbon which are having the different physical properties like melting point, boiling point, conductivity etc, and there is the presence of free electron in the graphite which makes the graphite to be conductive and due to the absence of free electrons in diamond it is also known as the bad conductor of electricity.