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 120o.

• The layers are held only by weak van der Waals’ forces, so they can slide over each other.

Graphite

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 109.5o.

• It is the hardest known natural substance

Diamond

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 (C60)

• 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

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• SiO2, 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 SiO2 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.