Diamond, graphite and lonsdaleite are allotropes of:

Allotropes are different physical forms of the same element, in the same physical state, in which the atoms are arranged differently. They have the same chemical properties but different physical properties. Carbon has many allotropes:

Diamond: each carbon atom is bonded to four other carbon atoms in a rigid three-dimensional tetrahedral network. This makes diamond the hardest natural substance. It is transparent, has a high refractive index (about 2.42) and does not conduct electricity, because it has no free electrons. It is used in jewellery and in cutting and drilling tools.

Graphite: each carbon atom is bonded to three others in flat hexagonal layers. The layers are held together by weak forces and can slide over each other, so graphite is soft and slippery. It is used as a lubricant and in pencil "lead". Because each atom has one free electron, graphite conducts electricity and is used to make electrodes.

Lonsdaleite: also called hexagonal diamond, it is a rare form found in meteorites, formed when graphite is subjected to great heat and pressure during impacts. It is named after the crystallographer Kathleen Lonsdale.

Fullerenes: cage-like molecules such as buckminsterfullerene (C60), shaped like a football. Its discovery in 1985 earned Curl, Kroto and Smalley the Nobel Prize in Chemistry in 1996.

Graphene: a single layer of graphite, one atom thick, extremely strong and an excellent conductor. Geim and Novoselov received the Nobel Prize in Physics in 2010 for work on it.

Carbon nanotubes are rolled sheets of graphene. Coal, charcoal and coke are amorphous forms of carbon.

Why not the other options?

• ✗Silicon does not form diamond or graphite.
• ✗Sulphur has allotropes such as rhombic and monoclinic sulphur.
• ✗Phosphorus has allotropes such as white, red and black phosphorus.