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Covalent Bonding

Key Concepts

  • A covalent bond is formed when two atoms share a pair of electrons.

  • Covalently bonded substances can be either:

    1. molecular substances

    2. three dimensional covalent networks

  • A molecular substance contains discrete molecules with weaker intermolecular forces attracting one molecule to another.

  • A network covalent substance contains only strong covalent bonds between atoms.

  • Molecular solids have much lower melting points than covalent network solids.

Covalent Network Solids

The elements carbon, silicon and boron form covalent networks instead of covalent molecules.

Silicon dioxide, SiO2, also exists as a covalent network and is known as quartz. Its structure is similar to diamond.

Carbon

Carbon forms 2 naturally occurring covalent network solids:

  • graphite

  • diamond
graphite diamond
structure
bonding each carbon atom makes 3 covalent bonds each carbon atom makes 4 covalent bonds
structure carbon atoms in a hexagonal arrangement forming layers carbon atoms in a tetrahedral arrangement
melting point very high (sublimes at ~3500K) due to the large amount of energy required to break strong covalent bonds very high (~4000K) due to the large amount of energy required to break strong covalent bonds
electrical conductivity good conductor between the carbon layers due to delocalised electrons between the carbon layers insulator due to no delocalised electrons (all electrons are used in covalent bonding)
hardness soft (1-2 on Mohs scale) because the carbon layers can slide over each other hardest known natural mineral (10 on Mohs scale)
colour black colourless
uses lubricant due to its softness abrasive due to its hardness


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