Graphene and carbon allotropes: structure and properties
Bonding, structure, and the properties of matter • Structure and bonding of carbon
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Atomic structure and bonding in graphene
Each carbon atom in graphene is sp2 hybridised, producing three coplanar sigma bonds that form a robust hexagonal lattice. The remaining unhybridised p-orbital on each carbon overlaps sideways with neighbours, creating a system of delocalised pi electrons that extends over the whole sheet. The sheet is one atom thick, producing a two-dimensional network where each bond length is equal due to delocalisation, and bond angles are approximately 120 degrees. Delocalised electrons provide charge carriers that move freely across the lattice. The strong sigma bonds between carbon atoms give the sheet high tensile strength and stability. Layer stacking produces graphite; a single isolated layer defines graphene.
Properties explained by structure and bonding
Electrons delocalised across the sheet allow graphene to conduct electricity in the plane because moving charge carriers require a continuous network of overlapping orbitals. Strong covalent sigma bonds between carbon atoms cause very high tensile strength because applied force must break many robust bonds to fracture the lattice. High thermal conductivity arises because vibrations (phonons) travel efficiently through the continuous covalent network. Thinness and delocalisation cause optical transparency since a single layer absorbs only a small fraction of visible light. Extremely large surface area per unit mass arises because each atom lies at the surface, which affects chemical reactivity and adsorption. Impermeability to standard gases results from the dense hexagonal bonding that leaves no continuous pathway for small molecules.
Recognising graphene in diagrams and descriptions
Graphene appears as a single flat hexagonal lattice made of hexagons with alternating shared edges; diagrams show one-atom-thick sheets with each carbon at a vertex of hexagons. Bonding annotations show sp2 hybridisation or three sigma bonds per carbon and delocalised pi electrons represented as a cloud or arrows above and below the plane. Descriptions emphasize a single layer, two-dimensional structure, equal bond lengths, and high electron mobility. Differences from graphite appear in layer count: graphene is a single layer, while graphite consists of many stacked layers held by weak van der Waals forces. Graphene diagrams lack pentagons or closed spherical motifs, which distinguishes graphene from fullerene structures.
Fullerenes: structure, bonding, and recognition
Fullerenes are molecular allotropes of carbon composed of closed cages of hexagons and pentagons. Carbon atoms in fullerenes are sp2 hybridised and form three sigma bonds, while delocalised pi electrons extend over the curved surface but are less uniformly delocalised than in a flat sheet. Presence of pentagonal rings introduces curvature, causing the sheet to close and form spheres, ellipsoids, or tubes. Diagrams of fullerenes show closed cages (for example C60 resembles a soccer ball) with both pentagonal and hexagonal faces. Descriptions highlight discrete molecules rather than extended sheets, molecular solubility in some organic solvents, and properties such as electron-accepting behaviour and distinctive vibrational modes caused by curvature.
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