Diamond structure and properties explained

Bonding, structure and the properties of matter • Structure and bonding of carbon

Flashcards

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What happens to diamond when heated in air at high temperature?

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Diamond oxidises (burns) to form carbon dioxide at sufficiently high temperatures in air.

Key concepts

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Atomic arrangement and bonding

Each carbon atom in diamond forms four single covalent bonds directed towards the corners of a tetrahedron. The bonds result from sp3 hybridisation of the carbon orbitals. The tetrahedral units link together to form an extensive, continuous three-dimensional giant covalent lattice.

Giant covalent lattice

The lattice contains atoms joined by strong covalent bonds throughout the solid. A very large amount of energy is required to break many covalent bonds, so diamond has a very high melting point and boiling point. The network structure lacks discrete molecules, so diamond does not melt or boil easily under normal conditions.

Lack of free charge carriers

All valence electrons in diamond pair in covalent bonds and localise between atoms. The absence of delocalised electrons prevents electrical conduction through the lattice, so diamond behaves as an electrical insulator. Substitutional impurities (for example boron) provide charge carriers and can make diamond semiconducting or electrically conductive.

Mechanical hardness and brittleness

Strong covalent bonds in every direction produce extreme hardness, because many bonds must be broken to create a permanent indentation. Directional bonds and the rigid lattice also allow cracks to propagate along certain planes, producing brittleness and cleavage despite the high hardness.

Thermal conductivity and optical properties

Strong bonds between light carbon atoms enable efficient transfer of vibrational energy (phonons) across the lattice, so diamond conducts heat very well. A wide electronic band gap prevents visible-light absorption, producing transparency. A high refractive index and strong dispersion lead to the characteristic sparkle of gem-quality diamond.

Key notes

Important points to keep in mind

Each carbon atom forms four covalent bonds (sp3), producing a tetrahedral geometry.

Diamond is a giant covalent lattice rather than a molecular substance.

All valence electrons are localised in bonds, so diamond is an electrical insulator except when doped.

Strong covalent bonds throughout the lattice cause extreme hardness and a very high melting point.

Directional bonding makes diamond hard but also gives cleavage planes that cause brittleness.

Efficient phonon transfer and light atomic mass cause high thermal conductivity.

Diamond remains insoluble because solvents cannot break the extensive covalent network.

Impurities or defects can change electrical behaviour (for example boron-doped diamond becomes conductive).

Diamond oxidises to carbon dioxide at high temperatures in air.

Gem-quality optical effects arise from transparency, high refractive index and dispersion.