Topic 4

Allotropes and Alloys

Allotropes

• Allotropes are different structural modifications of the same element.
• They vary in physical and chemical properties.

Allotropes of carbon

• Graphite
• Diamond
• Graphene
• C₆₀ fullerene

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Covalent Network Solids

• Graphite, diamond and graphene are covalent network solids. (a structure in which the atoms are held together by covalent bonds in a giant three-dimensional lattice structure)
• C₆₀ fullerene in molecular.

Graphite

Graphite

• Three layers of hexagonal rings consisting of carbon atoms.
• Layers connected by weak intermolecular forces (London forces)
• Each carbon is trigonal planar, covalently bonded to another three carbons at 120 degrees.
• Coordination number of each carbon is 3.
• The covalent bonds between the carbons are strong, but the London forces are weak and so the sheets slide past each other.
• Properties: used as a lubricant, good conductor of electricity because it has delocalized electrons.

Diamond

Diamond

• Each carbon is covalently bonded to four other carbon atoms in a tetrahedral arrangement. (109.5 degrees)
• Coordination number is 4
• One of the hardest substances known because of the interlocking structural arrangement.
• Used as heavy-duty cutting tool
• Melting and boiling points are very high.
• No delocalization of electrons and therefore do not conduct electricity.
• Insoluble in all solvents because of strong covalent bonds.

Graphene

Graphene

• First two-dimensional crystal ever discovered.
• Single planar sheet of carbon atoms arranged hexagonally and only one atom in thickness.
• Each carbon bonded to three other carbons
• Coordination number is 3
• Carbon atoms are densely packed in honeycomb crystalline lattice.
• Excellent thermal and electrical conductor
• Great strength, flexibility and transparency.
• Used to replace metals in aerospace industry (low density and high strength)
• Used in LCD (liquid crystal display)

C₆₀ fullerene

Carbon arranged in closed shells (shape of a soccer ball) spherically symmetrical.

20 hexagonal surfaces and 12 pentagonal surfaces.

Each carbon covalently bonded to three others.

Coordination number is 3.

Arrangement is not planar.

Composed of individual molecules with strong covalent bonds but with weak London forces between the molecules.

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C₆₀ fullerene

• Black solids that do not dissolve in water.
• Can dissolve in some non polar solvents
• Do not conduct electricity.
• They do possess delocalized electrons but they cannot move from one molecule to another in the structure.
• Used in superconductivity
• Often referred to as buckyballs.

Silicon dioxide (quartz)

Silicon dioxide (quartz)

• Often found in its amorphous structure (solid with no ordered structure) as sand.
• Most crystalline form is quartz.
• Arrays of SiO₄ tetrahedrally arranged in a lattice.
• Each Si is bonded to 4 oxygen atoms and each oxygen atom is bonded to 2 silicon atoms.
• Arrangement is bent because of 2 non-bonding pairs of electrons oneach oxygen atom.
• High melting and boiling points because of strong covalent bonds.
• Insoluble in water
• Solid does not conduct electricity (no delocalized electron) or heat.
• Conduct in the molten state as the electrons are more free to move.

General properties of covalent network solids

• Have high melting points (typically greater than 1000 degrees and much higher than molecular substances)
• Poor electrical conductors except for graphite and graphene
• Typically insoluble in common solvents
• Generally hard, except for graphite with its layers which can slide past each other.

Common alloys