Allotropes of carbon

Same atoms, different substances

Pencil lead and a diamond ring look nothing alike. Pencil lead is soft, grey and black, and a diamond is clear and the hardest natural substance. Yet both are made only of carbon atoms. The different forms of the same element are called allotropes.

Diamond

In diamond, each carbon atom is bonded to four other carbon atoms, in a rigid, three dimensional network.

• It is the hardest natural substance.
• It has a very high melting point.
• It does not conduct electricity, because all the outer electrons are used in bonds.
• It shines, because it bends light strongly.
• Uses: jewellery, and cutting and drilling tools.

Graphite

In graphite, each carbon atom is bonded to three others, in flat layers of hexagons. The fourth electron of each atom is free to move.

• The layers are held together weakly, and they slide over one another. So graphite is soft and slippery.
• It conducts electricity, because of the free electrons.
• Uses: pencil leads, lubricant for machines, and electrodes in batteries and in the electrolysis of salt.

Fullerenes

A fullerene is a hollow ball of carbon atoms. The first to be found was C60, with 60 atoms joined in 12 pentagons and 20 hexagons, like a football. It was found in 1985 and named buckminsterfullerene, because its shape is like the domes of the architect Buckminster Fuller. Other forms are the carbon nanotubes and graphene, a single layer of graphite.

You should find that every fullerene has exactly $12$ pentagons. Only the number of hexagons changes, and $\text{C}_{60}$ has $20$ of them.

Amorphous carbon

Coal, coke, charcoal and soot are carbon with no regular structure. They are used as fuels and in industry.

All of them burn to the same product

Whichever form we burn in plenty of oxygen, the product is carbon dioxide.

Practise with new numbers

Try it in Python

This program lists the properties of the three forms, and it counts the atoms in $7.2\ \text{g}$ of C60.

Remember these

• Allotropes are different forms of the same element. Diamond, graphite and fullerene are allotropes of carbon.
• Diamond: four bonds, hardest. Graphite: three bonds, layers, soft and conducts. C60: a ball of $60$ atoms.
• All burn to carbon dioxide.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Why is diamond used to cut glass?

2. Why is graphite used to oil the parts of machines?

3. How many hexagons are there in $\text{C}_{60}$?

4. A diamond is burnt in pure oxygen. What is the only product?