Covalent bonding (AQA GCSE Chemistry: Combined Science)

What is covalent bonding?

• Non-metal atoms can share electrons with other non-metal atoms to obtain a full outer shell of electrons
• When two atoms share pairs of electrons, they form covalent bonds
• Covalent bonds between atoms are very strong
• When two or more atoms are covalently bonded together, they form ‘molecules’
• Covalently bonded substances may consist of small molecules or giant molecules
• Weak intermolecular forces exist between individual molecules
  • For example, in methane, each molecule consists of four hydrogen atoms covalently bonded to a carbon atom, and in between individual methane 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

Covalent bonding

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

• Covalent substances tend to have small molecular structures, such as Cl₂, H₂O or CO₂
• These small molecules are known as simple molecules
• Small covalent molecules can be represented by dot and cross diagrams
• You need to be able to describe and draw the structures of the following molecules using dot-and-cross diagrams: hydrogen (H₂), chlorine (Cl₂), oxygen (O₂), nitrogen (N₂), hydrogen chloride (HCl), water (H₂O), ammonia (NH₃) and methane (CH₄)
• The correct dot and cross diagrams for these molecules are shown below:

Covalent bonding examples

Hydrogen, H₂

Dot & cross representation of a molecule of hydrogen

Chlorine, Cl₂

Dot & cross representation of a molecule of chlorine

Oxygen, O₂

Dot & cross representation of a molecule of oxygen

Nitrogen, N₂

Dot & cross representation of a molecule of nitrogen

Hydrogen chloride, HCl

Dot & cross representation of a molecule of hydrogen chloride

Water, H₂O

Dot & cross representation of a molecule of water

Ammonia, NH₃

Dot & cross representation of a molecule of ammonia

Methane, CH₄

Dot & cross representation of a molecule of methane

• Not all covalent molecules are small; covalent molecules can also be very large
• For example, polymers and giant covalent structures
• Common polymers include polythene which is used extensively in plastic bags and polyvinyl chloride (PVC) which has many industrial applications, most notably in the production of water pipes.

Formation of polyethene

You should be able to represent the covalent bonds in a variety of molecules, including simple molecules (ethene) and polymers (polyethene)

• Some covalently bonded substances have giant covalent structures, such as graphite, diamond, and silicon dioxide
• These substances form giant crystal structures made from many atoms held together by covalent bonds

Dot and Cross Diagrams

• Advantages:
  • Useful for illustrating the transfer of electrons
  • Indicates from which atom the bonding electrons come from

• Disadvantages:
  • Fails to illustrate the 3D arrangements of the atoms and electron shells
  • Doesn’t indicate the relative sizes of the atoms

Ball and Stick Model

• Advantages:
  • Useful for illustrating the arrangement of atoms in 3D space
  • Especially useful for visualizing the shape of a molecule

• Disadvantages:
  • Fails at indicating the movement of electrons
  • The atoms are placed far apart from each other, which in reality is not the case as the gaps between atoms are much smaller

Ball and stick model

Ball and stick model of ammonia which illustrates the 3D arrangement of the atoms in space and the shape of the molecule

2D Representations of Molecules

• Advantages:
  • Displayed formulae are 2D representations and are basically simpler versions of the ball and stick model
  • Adequately indicate what atoms are in a molecule and how they are connected

• Disadvantages:
  • Fail to illustrate the relative sizes of the atoms and bonds
  • Cannot give you an idea of the shape of a molecule and what it looks like in 3D space

2D representation of ammonia

Displayed formula of ammonia