- A covalent bond is formed when two atoms share a pair of electrons.
- Covalently bonded substances can be either:
- molecular substances
- 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 forms 2 naturally occurring covalent network solids:
||each carbon atom makes 3 covalent bonds
||each carbon atom makes 4 covalent bonds
||carbon atoms in a hexagonal arrangement forming layers
||carbon atoms in a tetrahedral arrangement
||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
||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)
||soft (1-2 on Mohs scale) because the carbon layers can slide over each other
||hardest known natural mineral (10 on Mohs scale)
||lubricant due to its softness
||abrasive due to its hardness