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Allotropes of Carbon

Allotropes of Carbon. Topic 4.2. Covalent Crystalline Solids.

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Allotropes of Carbon

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  1. Allotropes of Carbon Topic 4.2

  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.

  3. 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

  4. 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.

  5. Graphite

  6. 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

  7. 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

  8. Diamond

  9. 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.

  10. 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

  11. Fullerene

  12. 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.

  13. 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

  14. 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

  15. 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.

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