Which of the following is the hardest substance known?

The correct answer is Diamond.

Key Points

• Diamond is the hardest substance on Earth.
• It has carbon atoms with four covalent bonds (four valence electrons is connected with four other carbon atoms).
• Diamond is the purest form of carbon.
• It is a non-metal that reflects light.
• It is a good insulator(bad conductor) of heat and electricity.
• It is used to drill rocks and to cut glasses.
• Diamond has a high density and high refractive index of 2.415.

Important Points

Allotropes

Substances with the same chemical properties but different physical properties are called allotropes.

Some of the allotropes of carbon are

• Diamond
• Graphite
• Buckminsterfullerene
• Charcoal

Additional Information

Buckminsterfullerene

It is a crystalline form of carbon.

• It was named after an American architect, Buckminster fuller.
• It consists of 60 carbon atoms joined to form a spherical molecule similar to a soccer ball. 
• The formula of 'Buckminsterfullerene' is C60.

Graphite

• It is chemically more reactive than diamond.
• It is a good conductor (bad insulator) of heat and electricity.
• Graphite has a hexagonal structure held by weak Vander Waal's force.

Uses:

• It is used as an electrode in furnaces.
• In a nuclear reactor, graphite is used as a moderator.
• It helps as a reducing agent in the steel manufacturing industry.

Iron

• Iron is the metal present in group 8 of the periodic table.
• Iron combined with nickel and chromium forms stainless steel.
• Wrought iron is known as the purest form of iron.
• In 2021, Karnataka became the largest producer of iron ore in India.​​