Covalent bonding

Covalent bonding involves a shared pair of electrons between non-metal atoms, held together by the electrostatic attraction between the positive nuclei and the negative shared pair.

Dot-and-cross diagrams represent this bonding for diatomic molecules (H₂, O₂, N₂, Cl₂, HCl), inorganic molecules (H₂O, NH₃, CO₂), and organic molecules (CH₄, C₂H₆, C₂H₄, and halogen-containing molecules with up to two carbon atoms).

Bonds Formed by Each Element

• Each carbon atom always forms four covalent bonds
• Oxygen forms two covalent bonds
• Nitrogen forms three covalent bonds
• Hydrogen forms one covalent bond

Simple molecular substances

• Have low melting and boiling points because the weak intermolecular forces between molecules require little energy to overcome — the strong covalent bonds within the molecules are not broken
• Boiling points generally increase with relative molecular mass because larger molecules have stronger intermolecular forces

Diamond

• Has a giant covalent structure with very high melting points because many strong covalent bonds must be broken
• Is hard and non-conducting because all four electrons per carbon are used in bonds

Silicon dioxide

• Has a giant covalent structure with very high melting points because many strong covalent bonds must be broken

Graphite

• Has a giant covalent structure with very high melting points because many strong covalent bonds must be broken
• Conducts electricity because each carbon uses only three electrons in bonds, leaving one delocalised electron per atom free to carry charge

C₆₀ fullerene

• Is a simple molecular substance with a lower melting point than diamond or graphite

Covalent compounds do not usually conduct electricity because they lack free electrons or ions.

At a Glance

Key Definition Covalent bonding involves a shared pair of electrons between non-metal atoms, held together by the electrostatic attraction between the positive nuclei and the negative shared pair.