4.3 - Giant covalent structures : Allotropes of Carbon

Allotropes

Occur when an element can exist with more than one structure in the same physical state

Examples of allotropes of C

* Diamond
* Graphene
* Graphite
* C60 - Bucky balls

Diamond

* All 4 bonds are equally strong
* Sigma bonds
* Tetrahedron
* Sp3 hybridised
* Giant covalent structure; extremely hard to break down
* Strong covalent bonds; no weak intermolecular forces
* M.p is high
* Diamond is one of the hardest known substances and is used industrially in drills and polishing tools

Graphite

* 3 very strong sigma covalent bonds
* Layer of interlocking hexagons
* Sp2 hybridised
* Other electron exist in a p-orbital
* This can overlap with other p-orbitals forming a pi cloud above and below the layer
* Strong covalent bonds in layers of hexagons
* Weak intermolecular forces layers together
* Explains why graphite is used as a lubricant
* Layers of hexagon can easily be scrapped off
* Good conductor of electricity; delocalised ‘p’ electrons able to move throughout the layer of hexagons

Graphene

* Single layer of graphite
* Planar C atoms arranged hexagonally
* C bonded to 3 other C atoms with covalent bonds
* Sp2 hybridised (1 delocalised electron)
* Giant covalent structure
* 2D crystalline lattice
* Very good conductor of electricity; delocalised p electrons
* Strong, flexible and transparent
* Most C reactive, reactive edges due to unoccupied ‘dangling’ bonds

C60 or Bucky balls

* Family of spherical molecules arranged in **pentagons and hexagons** like a football
* C atom bonded in 20 hexagons and 12 pentagons
* 3 strong sigma bonds arranged in pentagons and hexagons
* Perfectly join up to form the ball shape
* Sp2 hybridised
* Makes them extremely strong yet lightweight
* Weak LDF hold molecules together so the m.p is much lower than diamond and graphite
* Un-bonded electron exist in a p-orbital forming a pi cloud over the molecule
* Conducts electricity but not as well as the layered graphite structure

Silicon

* Silicon atoms are arranged the same way as C atoms in diamond
* 4 sigma bonds
* Sp3 hybridised
* Does not show allotropic behavior
* Covalent bonding predominates throughout

Silicon dioxide

* Contains Si atoms bonded covalently to 4 O2 atoms
* Tetrahedral arrangement
* O2 atoms bonded covalently to 2 Si atoms
* Covalently bonded giant structures
* High m.p
* Does not conduct electricity since sp3 hybridised