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Comparison between diamond and graphite

Diamond and graphite are both giant covalent substances made entirely of carbon atoms. Both have chemical formula $$\ce{C}$$, but they do not have a molecular formula.

Diamond and graphite are allotropes of carbon. Allotropes are different forms of the same element due to differences in structure.

Molecular structure

  • Diamond: Giant covalent structure, with each carbon covalently bonded to four other carbon atoms in a tetrahedral arrangement to form a rigid structure.
  • Graphite: Giant covalent structure, with each carbon covalently bonded to three other carbon atoms in a hexagonal arrangement.

Hardness

  • Diamond: Extremely hard. Due to rigid, tetrahedral arrangement of carbon atoms.
  • Graphite: Soft. Layers of hexagonally arranged carbon atoms can slide over one another, as the layers are held together by van der Waals forces of attraction.

Melting and boiling points

  • Diamond and graphite: Very high. A large amount of energy is required to break numerous, strong covalent bonds between carbon atoms.

Electrical conductivity

  • Diamond: Insulator. Mobile electrons are absent. All four valence electrons are used in covalent bonds.
  • Graphite: Conductor. Three out of four valence electrons are used for covalent bonding with other carbon atoms. Remaining valence electrons can be delocalised across the planes of carbon atoms.