Ionic Compounds — Properties and Structure

This lesson covers the structure of ionic compounds and how it explains their physical properties, as required by AQA GCSE Combined Science Trilogy (8464), section 4.2.1. You need to understand the giant ionic lattice, explain why ionic compounds have high melting points, and know when they conduct electricity.

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The Giant Ionic Lattice

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:

• Each positive ion is surrounded by negative ions, and vice versa.
• The electrostatic forces of attraction act in all directions — every ion attracts all nearby ions of opposite charge.
• There are no individual molecules — the formula (e.g. NaCl) gives the simplest ratio of ions.

graph TD
    A["Giant Ionic Lattice"] --> B["Regular repeating<br/>3D arrangement"]
    A --> C["Strong electrostatic<br/>forces in ALL directions"]
    A --> D["No individual<br/>molecules"]
    B --> E["Each ion surrounded<br/>by oppositely charged ions"]
    C --> F["High melting and<br/>boiling points"]

    style A fill:#2c3e50,color:#fff
    style B fill:#2980b9,color:#fff
    style C fill:#2980b9,color:#fff
    style D fill:#2980b9,color:#fff
    style E fill:#27ae60,color:#fff
    style F fill:#27ae60,color:#fff

Sodium Chloride Lattice

In the sodium chloride lattice:

• Each Na⁺ ion is surrounded by 6 Cl⁻ ions.
• Each Cl⁻ ion is surrounded by 6 Na⁺ ions.
• The formula NaCl tells us the ratio is 1:1, but the actual structure extends in all directions.

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Properties of Ionic Compounds

1. High Melting and Boiling Points

Ionic compounds have high melting and boiling points because:

• The electrostatic forces of attraction between oppositely charged ions are very strong.
• These forces act in all directions throughout the lattice.
• A large amount of energy is required to overcome these many strong forces.

Exam Tip (AQA 8464): When explaining high melting points, always mention: (1) the forces are electrostatic attractions between oppositely charged ions, (2) they are strong, and (3) a large amount of energy is needed to overcome them. Simply saying "the bonds are strong" will not gain full marks — you must specify the type of force.

Comparing melting points:

Compound	Ions	Melting Point (°C)	Reason
Sodium chloride	Na⁺, Cl⁻	801	Strong electrostatic forces
Magnesium oxide	Mg²⁺, O²⁻	2852	Higher charges (2+ and 2−) create even stronger forces
Potassium chloride	K⁺, Cl⁻	770	Similar charges to NaCl but larger ions

Exam Tip: When asked to compare melting points, mention the charge on the ions. Greater ionic charges give stronger electrostatic attractions. MgO (2+ and 2−) has a much higher melting point than NaCl (1+ and 1−).

2. Electrical Conductivity

State	Conducts?	Reason
Solid	No	Ions are held in fixed positions in the lattice and cannot move to carry a charge
Molten (liquid)	Yes	Ions are free to move and can carry a charge through the liquid
Dissolved in water	Yes	Ions are free to move in solution and can carry a charge

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 locked in place. When melted or dissolved, the lattice breaks down and the ions become mobile.

graph LR
    A["Solid ionic<br/>compound"] -->|"Heat / Dissolve"| B["Molten or<br/>aqueous solution"]
    A --> C["Ions in fixed<br/>positions — CANNOT<br/>conduct"]
    B --> D["Ions free to<br/>move — CAN<br/>conduct"]

    style A fill:#e74c3c,color:#fff
    style B fill:#27ae60,color:#fff
    style C fill:#e74c3c,color:#fff
    style D fill:#27ae60,color:#fff

Exam Tip: Do NOT say "electrons carry the charge" for ionic compounds. It is the ions that move to carry charge, not electrons. This is one of the most common errors students make.

3. Brittleness

Ionic compounds are brittle — they shatter when hit rather than bending. This is because:

• A force can cause layers of ions to slide slightly.
• This brings ions of the same charge next to each other.
• These like-charged ions repel each other strongly.
• The lattice shatters as a result.

4. Solubility

Many ionic compounds are soluble in water because:

• Water molecules are polar (they have a slight positive end and a slight negative end).
• The polar water molecules can surround and separate individual ions from the lattice.
• The water molecules form attractions with the ions, providing enough energy to overcome the lattice forces.

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Structure vs Properties: Summary Table

Property	Explanation
High melting/boiling point	Strong electrostatic forces between oppositely charged ions require a large amount of energy to overcome
Does not conduct when solid	Ions held in fixed positions — cannot move to carry charge
Conducts when molten/dissolved	Ions free to move and carry charge
Brittle	Displacing layers brings like charges together, causing repulsion and shattering
Often soluble in water	Polar water molecules can separate ions from the lattice

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Common Mistakes

Mistake	Correction
"Ionic compounds have strong bonds" (too vague)	Specify "strong electrostatic forces of attraction between oppositely charged ions"
"Electrons carry the charge when ionic compounds conduct"	Ions carry the charge, not electrons
"Ionic compounds conduct in the solid state"	They only conduct when molten or dissolved — ions must be free to move
"NaCl is a molecule"	NaCl is the simplest ratio of ions in a giant ionic lattice — it is not a molecule
Confusing melting with boiling	Melting = solid → liquid; boiling = liquid → gas

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Summary