GCSE Chemistry revision

Covalent bonding

Chemical bonds, ionic, covalent and metallic

AQA 4.2.1.4 Foundation & Higher

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Read on screen, then print for Cornell-style active revision.

A covalent bond forms when two non-metal atoms share electrons so that both achieve a full outer shell.
Unlike ionic bonding, where electrons are transferred, covalent bonding involves sharing, making it ideal when both atoms need extra electrons.
For example, two chlorine atoms each share one electron, giving both a full outer shell of eight electrons.

1. Introduction AQA

A covalent bond forms when two non-metal atoms share electrons so that both achieve a full outer shell.
Unlike ionic bonding, where electrons are transferred, covalent bonding involves sharing, making it ideal when both atoms need extra electrons.
For example, two chlorine atoms each share one electron, giving both a full outer shell of eight electrons.

A dot and cross diagram shows the electrons in the outer shells of bonded atoms, using dots for one atom and crosses for the other.
This method clearly shows which electrons originally belonged to which atom and how they are shared in the covalent bond.
You may be asked to draw all electron shells or just the outermost shell, so always check what the question requires.

2. Dot and Cross Diagrams AQA

A dot and cross diagram shows the electrons in the outer shells of bonded atoms, using dots for one atom and crosses for the other.
This method clearly shows which electrons originally belonged to which atom and how they are shared in the covalent bond.
You may be asked to draw all electron shells or just the outermost shell, so always check what the question requires.

A displayed formula uses the chemical symbols of atoms joined by straight lines, where each line represents one covalent bond.
Displayed formulae are much easier to draw for large, complex molecules, such as glucose, which would be too complicated as a dot and cross diagram.
The main limitation of displayed formulae is that they do not show the three-dimensional (3D) shape of the molecule.

3. Displayed Formulae AQA

A displayed formula uses the chemical symbols of atoms joined by straight lines, where each line represents one covalent bond.
Displayed formulae are much easier to draw for large, complex molecules, such as glucose, which would be too complicated as a dot and cross diagram.
The main limitation of displayed formulae is that they do not show the three-dimensional (3D) shape of the molecule.

3D models attempt to show how atoms are actually arranged in space, giving a more realistic representation of a molecule.
You are not required to predict 3D shapes at GCSE, but you should be able to recognise common 3D molecular models.

Ammonia has the molecular formula NH₃, meaning it contains one nitrogen atom and three hydrogen atoms.
Each hydrogen atom needs one extra electron to fill its first energy level (maximum of 2 electrons), while nitrogen needs three extra electrons to fill its second energy level (maximum of 8 electrons).
Each hydrogen shares one electron with the nitrogen, forming three covalent bonds and giving all atoms full outer shells.
In the displayed formula of ammonia, three lines are drawn from the nitrogen atom, one to each hydrogen atom.

5. Working Example: Ammonia (NH₃) AQA

Ammonia has the molecular formula NH₃, meaning it contains one nitrogen atom and three hydrogen atoms.
Each hydrogen atom needs one extra electron to fill its first energy level (maximum of 2 electrons), while nitrogen needs three extra electrons to fill its second energy level (maximum of 8 electrons).
Each hydrogen shares one electron with the nitrogen, forming three covalent bonds and giving all atoms full outer shells.
In the displayed formula of ammonia, three lines are drawn from the nitrogen atom, one to each hydrogen atom.

Simple molecular substances are small molecules in which atoms are held together by strong covalent bonds within the molecule.
Between individual molecules, there are only weak intermolecular forces, which are easily broken, giving these substances low melting and boiling points.
Key examples of simple molecular substances you should know include chlorine (Cl₂), ammonia (NH₃), water (H₂O), and methane (CH₄).
Polymers Polymers are large molecules made up of long chains of repeating units called monomers joined together by covalent bonds.
Common examples of polymers include plastics used in plastic bags and synthetic fibres used in t-shirts.

6. Simple Molecular Substances AQA

Simple molecular substances are small molecules in which atoms are held together by strong covalent bonds within the molecule.
Between individual molecules, there are only weak intermolecular forces, which are easily broken, giving these substances low melting and boiling points.
Key examples of simple molecular substances you should know include chlorine (Cl₂), ammonia (NH₃), water (H₂O), and methane (CH₄).
Polymers Polymers are large molecules made up of long chains of repeating units called monomers joined together by covalent bonds.
Common examples of polymers include plastics used in plastic bags and synthetic fibres used in t-shirts.

Giant covalent structures contain billions or trillions of atoms arranged in a regular lattice, all joined by strong covalent bonds.
Because every atom is covalently bonded to its neighbours throughout the entire structure, giant covalent substances are extremely strong with very high melting points.
Key examples of giant covalent structures include diamond, graphite, and silicon dioxide (SiO₂).

7. Giant Covalent Structures AQA

Giant covalent structures contain billions or trillions of atoms arranged in a regular lattice, all joined by strong covalent bonds.
Because every atom is covalently bonded to its neighbours throughout the entire structure, giant covalent substances are extremely strong with very high melting points.
Key examples of giant covalent structures include diamond, graphite, and silicon dioxide (SiO₂).