How do atoms join by ionic and covalent bonding, and how does the bonding explain a substance's properties?
Ionic bonding as the transfer of electrons forming a giant ionic lattice, covalent bonding as shared electron pairs in simple molecules, dot-and-cross diagrams, and using structure to explain melting points and electrical conductivity.
A focused CCEA GCSE Single Award Science answer on bonding, covering ionic bonding and the giant ionic lattice, covalent bonding in simple molecules, dot-and-cross diagrams, and how the structure explains melting points and electrical conductivity.
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What this dot point is asking
CCEA wants you to explain ionic bonding as electron transfer, covalent bonding as shared electron pairs, draw dot-and-cross diagrams, and use the structure of a substance to explain its melting point and whether it conducts electricity.
Ionic bonding
The charge equals the number of electrons lost or gained: Group 1 metals form +1 ions, Group 2 form +2, Group 7 non-metals form -1 and Group 6 form -2. The formula balances the charges, so magnesium chloride is .
This explains the properties:
- High melting and boiling points, because many strong forces must be overcome.
- Conduct only when molten or dissolved, because only then are the ions free to move and carry charge; in the solid they are locked in place.
Covalent bonding
The number of bonds an atom forms is usually the number of electrons it needs to fill its outer shell: hydrogen 1, chlorine 1, oxygen 2, nitrogen 3, carbon 4. A dot-and-cross diagram shows the shared pairs, using dots for one atom and crosses for the other, with only the outer electrons drawn.
So simple molecular substances have low melting and boiling points and do not conduct electricity, because they have no charged particles free to move.
Examples in context
Example 1. Why molten salt conducts but solid salt does not. Solid sodium chloride holds its ions tightly in a lattice, so no charge can move and it does not conduct. Heating it until it melts frees the ions, which can then drift to the electrodes and carry a current. This single idea, that conduction needs charged particles free to move, runs through the whole topic.
Example 2. Why oxygen is a gas but salt is a solid. Oxygen is a simple molecule with only weak forces between molecules, so very little energy is needed to separate them, and it is a gas at room temperature. Sodium chloride is a giant lattice with strong forces throughout, so it stays solid until about 800 degrees. Comparing the two shows how the type of structure, not the strength of the bonds inside molecules, decides the state.
Try this
Q1. What type of bond is a shared pair of electrons? [1 mark]
- Cue. A covalent bond.
Q2. Why does solid sodium chloride not conduct electricity? [1 mark]
- Cue. Its ions are locked in place and cannot move.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA SAS 20204 marksExplain, in terms of structure and bonding, why sodium chloride has a high melting point and only conducts electricity when molten or dissolved.Show worked answer →
Four marks for the lattice, the forces, and the conduction conditions.
Sodium chloride is a giant ionic lattice of positive and negative ions held by strong electrostatic forces acting in all directions.
To melt it, many of these strong forces must be overcome, which needs a lot of energy, so the melting point is high.
In the solid the ions are locked in place and cannot move, so it does not conduct.
When molten or dissolved the ions are free to move and carry charge, so it conducts. Markers reward the giant lattice, strong forces, and mobile ions only when molten or dissolved.
CCEA SAS 20193 marksExplain why simple molecular substances such as water have low melting and boiling points.Show worked answer →
Three marks for the bonds, the forces and the energy.
In a simple molecular substance the covalent bonds inside each molecule are strong, but the forces between separate molecules are weak.
When the substance melts or boils, only these weak intermolecular forces are overcome, not the strong covalent bonds.
Because only a little energy is needed to overcome the weak forces, the melting and boiling points are low.
Markers reward the contrast between strong covalent bonds and weak intermolecular forces, and that melting overcomes the weak forces.
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Sources & how we know this
- CCEA GCSE Science: Single Award specification — CCEA (2017)