Covalent Bonding (Edexcel GCSE Chemistry: Combined Science)

• Non-metal atoms can share electrons with other non-metal atoms to obtain a full outer shell of electrons
• When atoms share pairs of electrons, they form covalent bonds
• Covalent bonds between atoms are very strong
• When two or more atoms are chemically bonded together, they form ‘molecules’
• Covalently bonded substances may consist of small molecules or giant molecules
• Weak intermolecular forces exist between individual molecules
  • E.g. Each liquid water molecule consists of two hydrogen atoms covalently bonded to an oxygen atom, and in between two individual water molecules there are weak intermolecular forces

• Shared electrons are called bonding electrons and occur in pairs
• Electrons on the outer shell which are not involved in the covalent bond(s) are called non-bonding electrons
• Simple covalent molecules do not conduct electricity as they do not contain free electrons

Diagram showing covalent bonding in a molecule of chlorine (Cl₂)

The Size of Molecules

• Simple molecular substances consist of molecules which contain atoms that are bound tightly together by strong covalent bonds
• An atom is typically about 0.1 nanometres in size, which is 0.0000000001 m or 10^-10 m
• Simple molecules contain only a few atoms, so the sizes of atoms and simple molecules have similar ranges
• The smallest molecule is hydrogen, H_2, which is just 0.074 nm long or 7.4 x 10^-11 m
• Even though individual atoms and molecules are extremely small, developments in electron microscopy can produce images of atoms and simple molecules

Pentacene (C₂₂H₁₄) was first imaged in 2009 by the IBM Research team in Zurich using a technique called atomic force microscopy