A carbon atom has 6 electrons, with 2 in the K shell and 4 in the L shell. It is an element of group 14 of the periodic table.
The 4 outer electrons are called valence electrons, and a carbon atom shares electrons with other atoms to form 4 covalent bonds. Because carbon atoms can join together in many different arrangements, the element carbon exists in several forms with different properties.
In diamond, each carbon atom forms covalent bonds with 4 other atoms, and these bonds, pointing towards the corners of a regular tetrahedron, extend in three dimensions.
This makes diamond the hardest natural substance, and it is used as a gemstone and in tools for cutting glass and rock. All its valence electrons are used in bonds, so it does not conduct electricity.
In graphite, each carbon atom forms covalent bonds with 3 other atoms, making flat layers of regular hexagons. The layers are held on top of one another only by weak intermolecular forces, so they slide and peel off easily, and graphite is soft. This is why it is used in pencil leads.
The remaining valence electron of each atom can move along the layers, so graphite conducts electricity well and is used for purposes such as the electrodes of batteries.
Fullerene, \mathrm{C_{60}}, is a molecule in which 60 carbon atoms are joined in the shape of a ball made of 12 pentagons and 20 hexagons.
It was discovered in 1985, and the three researchers who discovered it received the Nobel Prize in Chemistry in 1996. It differs from diamond and graphite in being a molecule made of a fixed number of atoms.
When carbon burns in plenty of oxygen, it forms carbon dioxide, and when there is not enough oxygen, it forms carbon monoxide.
\mathrm{C} + \mathrm{O_2} \longrightarrow \mathrm{CO_2}
2\mathrm{C} + \mathrm{O_2} \longrightarrow 2\mathrm{CO}
Solid carbon dioxide is called dry ice, and at about -79 °C it turns into a gas without becoming a liquid. The white mist that appears around dry ice is made of tiny droplets of water, formed when water vapour in the air is cooled.
Carbon monoxide is a colourless, odourless, poisonous gas. It binds to the haemoglobin in blood much more strongly than oxygen does, so when it is breathed in, oxygen can no longer be carried around the body.