Key Stage 4 science · Chemistry – Structure, bonding and the properties of matter
GCSE ionic, covalent and metallic bonding: a parent’s guide
GCSE chemistry explains why materials behave as they do by looking at how their particles are held together. Students learn the three types of strong chemical bond, ionic, covalent and metallic, and the weaker forces between molecules, then use them to explain changes of state, melting points, hardness and electrical conductivity. The skill that earns marks is linking a property to the right particles and forces.
Bonding and properties of matter, explained
The particle model says that matter is made of tiny particles in constant motion. In a solid they vibrate in fixed positions, in a liquid they move past each other, and in a gas they move freely and far apart. When a substance is heated, energy is transferred to the particles, and at the melting or boiling point they have enough energy to overcome the forces holding them together. The size of those forces decides the temperature at which this happens. Importantly, the particles themselves do not melt, expand or change when a substance changes state. It is the arrangement and the forces between them that change.
In ionic bonding, a metal atom transfers one or more electrons to a non-metal atom. The atoms become oppositely charged ions, such as Na⁺ and Cl⁻, which attract each other very strongly. Ionic compounds form a giant lattice with the ions in a regular pattern, with strong electrostatic attraction in every direction. This gives them high melting and boiling points. They do not conduct electricity when solid, because the ions are held in position, but they do when molten or dissolved in water, because the ions are free to move and carry the charge.
In covalent bonding, atoms of non-metals share pairs of electrons. Simple molecules such as water, methane and carbon dioxide have strong covalent bonds inside each molecule, but only weak intermolecular forces between molecules. That is why they have low melting and boiling points, are often gases or liquids at room temperature, and do not conduct electricity, since they have no free charged particles. Larger molecules have stronger intermolecular forces, so their boiling points are higher. Some covalent substances, such as diamond and silicon dioxide, are giant structures in which every atom is held by strong bonds, so they have very high melting points.
Metallic bonding is found in metals and alloys. The atoms are arranged in layers, with the outer electrons delocalised, which means they are free to move through the whole structure. The attraction between the positive metal ions and the sea of delocalised electrons is strong, so metals usually have high melting points. The free electrons allow metals to conduct electricity and heat, and the layers can slide over each other, which is why metals can be bent and shaped. Students are asked to draw and label diagrams for all three kinds of bonding, such as dot-and-cross diagrams for ionic and covalent compounds, and to match each structure to its properties.
Words your child will hear
- Ion
- An atom or group of atoms that has gained or lost electrons, so it has an electrical charge.
- Ionic bond
- The strong electrostatic attraction between oppositely charged ions.
- Covalent bond
- A strong bond formed when two atoms share a pair of electrons.
- Intermolecular forces
- Weak forces of attraction between molecules, much weaker than the bonds inside them.
- Delocalised electrons
- Outer electrons in a metal that are free to move through the structure.
- Giant lattice
- A huge, regular, repeating arrangement of particles, as in sodium chloride.
- Change of state
- A change between solid, liquid and gas, such as melting, boiling or condensing.
What the National Curriculum says
The programme of study for Key Stage 4 says:
- Students should be taught about changes of state of matter in terms of particle kinetics, energy transfers and the relative strength of chemical bonds and intermolecular forces
- Students should be taught about types of chemical bonding: ionic, covalent, and metallic
- Students should be taught about bulk properties of materials related to bonding and intermolecular forces
Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.
Try it: a model answer on melting points
Sodium chloride melts at about 801 °C. Methane melts at about −182 °C. Explain the difference in terms of structure and bonding.
- Name the type of structure for each. Sodium chloride is an ionic compound with a giant lattice of Na⁺ and Cl⁻ ions. Methane is a simple molecular substance made of separate molecules, each with covalent bonds inside.
- For sodium chloride, describe the forces. Each ion is held by strong electrostatic attraction to the oppositely charged ions around it, in every direction through the lattice.
- Link to energy. A great deal of energy is needed to overcome these strong forces and separate the ions enough to form a liquid, so the melting point is high.
- For methane, name what is overcome. When methane melts, the covalent bonds inside the molecules stay intact. Only the weak intermolecular forces between molecules are overcome.
- Link to energy. Little energy is needed to overcome weak forces, so the melting point is very low, and methane is a gas at room temperature.
Sodium chloride has strong forces between ions throughout a giant lattice, so a lot of energy is needed to melt it. Methane has only weak intermolecular forces to overcome, because its covalent bonds do not break.
Common mistakes and how to help
Saying that covalent bonds break when a simple molecular substance melts or boils
Covalent bonds are strong, so students think they must be the reason for a high boiling point. Water boiling gives steam which is still H₂O, so the bonds inside molecules are intact.
What helps: Ask what the bubbles in boiling water contain. They are water molecules in the gas state, not hydrogen and oxygen. Only the forces between molecules were overcome.
Calling sodium chloride a molecule, or saying that ions in a solid are “free to move”
Everyday language uses “molecule” for any tiny particle, and a grain of salt does not look like a lattice of separate ions. In a solid lattice the ions vibrate around fixed positions and cannot carry charge through the crystal.
What helps: Ask when an ionic compound conducts, and why. A good answer says it is when molten or dissolved, because the ions are then free to move.
Believing that particles themselves expand, melt or get hotter when a substance changes state
A visible change in a lump of material is easy to imagine as a change in the particles. But the particles keep the same size and shape. It is their energy, spacing and the forces between them that matter.
What helps: Draw the particle diagrams for a solid, liquid and gas. Check that the particles are the same size in each, and that only their arrangement and movement differ.
Thinking that electrons carry the current in molten ionic compounds, or that metals conduct because of their ions
Metals and molten salts both conduct, so the explanations get mixed. In metals the carriers are delocalised electrons. In molten or dissolved ionic compounds the carriers are ions.
What helps: Make a two-line summary your child can say aloud: “metals conduct through electrons, molten salts through ions”. Then ask why a solid salt does not conduct.
How to help at home
- Model bonding with a kitchen kit. Use small balls of modelling clay or sweets joined by straws or sticks to build a methane molecule, then a block of alternating colours to represent a lattice. Compare how many links each particle has in each model.
- Explore changes of state safely. Put an ice cube, a little butter, a chocolate button and a coin on separate plates in a warm spot and see which melt, and in what order. The coin will not melt at all. Ask what the results suggest about the strength of the forces between particles in each. Any heating on a hob needs an adult.
- Test what conducts. With an adult, use a torch battery, a small torch bulb and two short wires to check whether a coin, a pencil lead, a plastic spoon and a dry salt crystal complete a circuit. Then ask why salt solution would be different. Do not use mains electricity.
- Two-minute explainers. Pick a substance, such as water, salt or copper, and ask your child to explain its melting point or conductivity out loud in under two minutes, using the words particles, forces and energy. Short spoken answers make a good rehearsal for exam questions.
Related topics
Builds on
- Key Stage 3: States of matter and the particle model
More on chemistry – Structure, bonding and the properties of matter
- Key Stage 4: Carbon chemistry and carbon structures
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