You are viewing a free preview of this lesson.
Subscribe to unlock all 10 lessons in this course and every other course on LearningBro.
This lesson covers the structure and properties of ionic compounds, as required by the AQA GCSE Chemistry specification (4.2.1). You need to understand what an ionic lattice is, why ionic compounds have high melting and boiling points, and why they can conduct electricity when molten or dissolved but not when solid.
Ionic compounds do not exist as individual pairs of ions. Instead, the ions are arranged in a regular, repeating three-dimensional pattern called a giant ionic lattice. In this lattice, every positive ion is surrounded by negative ions, and every negative ion is surrounded by positive ions. The strong electrostatic forces of attraction act in all directions — this is what makes the structure so stable.
The term giant means that the lattice extends in all three dimensions and contains billions upon billions of ions. There are no individual molecules in an ionic compound — the formula (e.g. NaCl) simply represents the ratio of ions present.
Exam Tip: When describing ionic lattice structures, always say "strong electrostatic forces of attraction between oppositely charged ions in all directions." The phrase "in all directions" is key — it distinguishes ionic bonding from a simple pair of ions and explains the high melting points.
Sodium chloride (NaCl) is the most commonly examined ionic compound. In its lattice:
graph TD
A["Giant Ionic Lattice of NaCl"] --> B["Na+ ions (positive)"]
A --> C["Cl- ions (negative)"]
B --> D["Each Na+ surrounded<br/>by 6 Cl- ions"]
C --> E["Each Cl- surrounded<br/>by 6 Na+ ions"]
D --> F["Strong electrostatic<br/>attraction in ALL<br/>directions"]
E --> F
F --> G["Very high melting<br/>and boiling points"]
style A fill:#2c3e50,color:#fff
style F fill:#e74c3c,color:#fff
style G fill:#27ae60,color:#fff
Ionic compounds have very high melting and boiling points. This is because there are strong electrostatic forces of attraction between the oppositely charged ions throughout the lattice. A large amount of energy is needed to overcome these forces and separate the ions, so a high temperature is required.
| Ionic Compound | Melting Point (degrees C) | Boiling Point (degrees C) |
|---|---|---|
| Sodium chloride (NaCl) | 801 | 1413 |
| Magnesium oxide (MgO) | 2852 | 3600 |
| Calcium fluoride (CaF2) | 1418 | 2533 |
| Potassium bromide (KBr) | 734 | 1435 |
Magnesium oxide has a much higher melting point than sodium chloride because Mg2+ and O2- ions have greater charges (2+ and 2-) than Na+ and Cl- (1+ and 1-). The greater the charge on the ions, the stronger the electrostatic forces between them, and the more energy is needed to overcome these forces.
Exam Tip: If asked to explain why MgO has a higher melting point than NaCl, you must refer to the greater ionic charges (2+ and 2- vs 1+ and 1-) leading to stronger electrostatic forces of attraction. Simply saying "stronger bonds" is not enough for full marks.
Ionic compounds can conduct electricity, but only under certain conditions.
| State | Can It Conduct? | Explanation |
|---|---|---|
| Solid | No | The ions are held in fixed positions in the lattice. They cannot move, so they cannot carry an electrical charge through the substance. |
| Molten (liquid) | Yes | When melted, the lattice breaks down and the ions are free to move. The ions can carry charge through the liquid, so it conducts electricity. |
| Dissolved in water | Yes | When dissolved, the lattice breaks apart and the ions are free to move in the solution. The ions carry charge through the solution, so it conducts electricity. |
The key idea is that for a substance to conduct electricity, it must contain charged particles that are free to move. In a solid ionic compound, the ions are charged but cannot move. When molten or dissolved, the ions become mobile.
Exam Tip: Always use the phrase "ions are free to move" when explaining electrical conductivity. Do not say "electrons are free to move" — that applies to metals, not ionic compounds. This is a very common mistake in exams.
Ionic compounds are brittle — they shatter when hit with force rather than bending. This is because when a force is applied, layers of ions slide over each other. When ions of the same charge come next to each other, the strong repulsion between them causes the lattice to split apart.
Many ionic compounds are soluble in water. Water molecules are polar (they have slightly positive and slightly negative ends), and they can surround individual ions, pulling them away from the lattice and dissolving the compound. However, some ionic compounds are insoluble — for example, barium sulfate and silver chloride — because the lattice energy is too great for water molecules to overcome.
The ability of ionic compounds to conduct electricity when molten or dissolved is the basis of electrolysis. In electrolysis, an electric current is passed through a molten or dissolved ionic compound, causing the ions to move to the electrodes:
This topic is covered in more detail in the electrolysis section of the AQA GCSE Chemistry specification.
Subscribe to continue reading
Get full access to this lesson and all 10 lessons in this course.