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.

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:

	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