Allotropes of Carbon Topic 4.2

Covalent Crystalline Solids There are substances which have a crystalline structure in which all the atoms are linked together by covalent bonds. Effectively, the crystal is a single molecule with a regular repeating pattern of covalent bonds, so is often referred to as a giant molecular structure or a macromolecule.

Allotropes of Carbon Allotropes are different forms of an element in the same physical state. Carbon has three allotropes: graphite, diamond, and fullerene. Different bonding within these structures give rise to distinct forms with different properties

Graphite In graphite, each carbon atom is covalently bonded to 3 others, forming hexagons in parallel layers with bond angles of 120 o. The layers are held only by weak van der Waals’ forces, so they can slide over each other.

Graphite

Density: 2.26 g cm -3 Contains one non-bonded, delocalized electron per atom, so it conducts electricity Non-lustrous, grey solid Used as a lubricant and in pencils

Diamond Each carbon atom is covalently bonded to 4 others, tetrahedrally arranged in a regular repeating pattern with bond angles of o. It is the hardest known natural substance

Diamond

Density: 3.51 g cm -3 All electrons are bonded; non conductor of electricity Lustrous crystal Polished for jewellery and ornamentation; used in tools and machinery for grinding and cutting glass.

Fullerene (C 60 ) Each carbon atom is bonded in a sphere of 60 carbon atoms, consisting of 12 pentagons and 20 hexagons. The structure is a closed spherical cage in which each carbon is bonded to 3 others

Fullerene

Density: 1.72 g cm -3 Easily accepts electrons to form ions Yellow crystalline solid Reacts with K to make superconducting crystalline material; related forms are used to make nanotubes for the electronics industry, catalysts and lubricants.

Silicon and Silicon Dioxide Like carbon, silicon is a Group 4 element and so its atoms have four valence shell electrons. In the elemental state, each silicon atom is covalently bonded to four others ina tetrahedral arrangement. This results in a giant lattice structure much like a diamond

Silicon Dioxide SiO 2, commonly known as silica or quartz, also forms a giant covalent structure. This is structure is also a tetrahedrally bonded structure with bonds between Si and O atoms. Each Si atom is covalently bonded to four oxygen atoms and each O to two Si Atoms

Silicon Dioxide Note the formula is SiO 2 and refers to the ratio of atoms within the giant molecule – the actual number of atoms present will be a very large multiple of this. The structure is strong, insoluble in water, has a high melting point, and does not conduct electricity or heat. These are all properties associated with glass and sand – different forms of silica.