What are different forms of elemental carbon?

Answer 1

Carbon has two naturally occurring pure forms (allotropes), which are graphite and diamond.

Graphite is a silver grey powder which consists of planes of carbon atoms arranged in interlocking hexagons. There are two kinds of bonds within the sheets, covalent and delocalised. The delocalised bond gives graphite its electrical conductivity. There are only weak forces between the sheets of atoms, which lets them slide easily over each other. This is why graphite is used as a lubricant, especially where liquids are inappropriate. Graphite is used to make the brushes for electrical motors. More obviously it is the main ingredient in the so called 'lead' of a pencil. (It's not, and never has been made of lead, the confusion arose because they look similar, and they both mark paper.)

Diamond has a three dimensional structure where each atom is bound to four others by covalent bonds. This is a very rigid structure and gives diamond its hardness. Diamond forms deep within the Earth, when the temperature and pressure are very high. It is used as a gemstone, but also in drill bits.

There are also artificially made forms of pure carbon, called fullerenes, commonly known as bucky balls, which have sixty or more atoms arranged in cage-like structures.

As well as these crystalline forms there are many amorphous and impure forms. Amorphous means that there is no regular arrangement of atoms. These include soot, lampblack, charcoal and coke. They tend to form when carbon compounds are heated in a poor supply of air.