How does ionic bonding form, and why do ionic compounds have their properties?
Ionic bonding as the transfer of electrons between metals and non-metals, the formation of ions, and the properties of ionic compounds.
A focused answer to the WJEC GCSE Science Double Award Unit 5 topic on ionic bonding, covering how electrons are transferred between metals and non-metals to form ions, the giant ionic lattice, and the properties of ionic compounds.
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What this dot point is asking
WJEC Double Award Unit 5 wants you to describe ionic bonding as the transfer of electrons, the formation of ions, and the properties of ionic compounds.
How ionic bonding forms
A metal atom loses its outer electrons to become a positive ion (cation); a non-metal atom gains those electrons to become a negative ion (anion). Both end up with a stable, full outer shell. For example, sodium (2,8,1) loses one electron to become (2,8), and chlorine (2,8,7) gains it to become (2,8,8).
The charges on ions
The compound is overall neutral, so the charges must balance (for example has one and two ).
The giant ionic lattice
The ions arrange themselves into a regular, repeating 3D structure called a giant ionic lattice, held together by strong electrostatic forces between the oppositely charged ions acting in all directions. This structure explains the properties of ionic compounds.
Drawing dot-and-cross diagrams
Ionic bonding is often shown with a dot-and-cross diagram, where the electrons from one atom are drawn as dots and those from the other as crosses, so you can see which atom each electron came from. For sodium chloride, you show the single outer electron leaving the sodium and joining the chlorine, giving a sodium ion (no outer electrons shown, charge ) and a chloride ion with a full outer shell (charge ), drawn in square brackets with the charge outside. Being able to draw or interpret a dot-and-cross diagram, and write the charges correctly, is a common exam skill for ionic bonding.
Why ionic compounds are brittle
Ionic compounds are usually brittle (they shatter when hit), which the lattice explains. If a force pushes the layers of ions so that like charges line up next to each other (positive next to positive), they repel, and the lattice splits apart along that plane. This is why an ionic crystal such as salt breaks cleanly rather than bending. Linking brittleness to the repulsion of like charges in the lattice is a higher-mark point that shows you understand how the structure causes the property.
Properties of ionic compounds
- High melting and boiling points: the many strong electrostatic forces need a lot of energy to break, so ionic compounds are solid at room temperature.
- Conduct electricity when molten or dissolved: the ions become free to move and carry charge.
- Do not conduct when solid: the ions are held in fixed positions and cannot move.
Try this
Q1. What charge does a Group 1 metal ion have? [1 mark]
- Cue. +1.
Q2. Why does molten sodium chloride conduct electricity? [1 mark]
- Cue. The ions are free to move and carry charge.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC style4 marksDescribe how an ionic bond forms between sodium and chlorine, including the charges of the ions formed.Show worked answer →
A Unit 5 describe question worth 4 marks. Reward: the sodium atom loses one electron from its outer shell (1) to become a sodium ion with a charge of +1 (1); the chlorine atom gains that electron (1) to become a chloride ion with a charge of -1 (1); the oppositely charged ions are held together by strong electrostatic attraction. Markers credit the transfer of an electron and the correct charges. A common error is to say the electrons are shared (that is covalent bonding).
WJEC style4 marksExplain why ionic compounds have high melting points and conduct electricity when molten but not when solid.Show worked answer →
A Unit 5 explain question worth 4 marks. Reward: ionic compounds have a giant lattice with strong electrostatic forces between the oppositely charged ions, so a lot of energy is needed to break them, giving a high melting point (2); they conduct when molten or dissolved because the ions are free to move and carry charge (1); they do not conduct when solid because the ions are held in fixed positions and cannot move (1). Markers credit the strong forces for the melting point and the free ions for conduction. A common error is to say electrons carry the charge.
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