Ionic Compounds

This lesson covers the structure and properties of ionic compounds as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand the giant ionic lattice structure and be able to explain the typical properties of ionic compounds in terms of their bonding and structure.

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The Giant Ionic Lattice

Ionic compounds do not exist as individual molecules. Instead, they form a giant ionic lattice — a regular three-dimensional arrangement of alternating positive and negative ions extending in all directions.

Key Features

• Every positive ion is surrounded by negative ions, and every negative ion is surrounded by positive ions.
• The ions are held together by strong electrostatic forces of attraction (ionic bonds) acting in all directions.
• The lattice is described as giant because it contains billions of ions arranged in a repeating pattern.

graph TD
    A["Giant Ionic Lattice<br/>(e.g. NaCl)"] --> B["Regular 3D arrangement"]
    A --> C["Alternating +/− ions"]
    A --> D["Strong electrostatic<br/>attractions in all<br/>directions"]

    style A fill:#2c3e50,color:#fff
    style B fill:#2980b9,color:#fff
    style C fill:#27ae60,color:#fff
    style D fill:#e67e22,color:#fff

Sodium Chloride Lattice

In sodium chloride (NaCl):

• Each Na⁺ ion is surrounded by 6 Cl⁻ ions.
• Each Cl⁻ ion is surrounded by 6 Na⁺ ions.
• This creates a cubic crystal structure visible even at macroscopic scale — salt crystals are cubic.

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Properties of Ionic Compounds

The properties of ionic compounds can be explained by their giant ionic lattice structure and the strong electrostatic forces between ions.

1. High Melting and Boiling Points

Property	Explanation
High melting point	A large amount of energy is needed to overcome the many strong electrostatic forces of attraction between oppositely charged ions throughout the lattice
High boiling point	Even more energy is needed to completely separate the ions from each other

Example values:

Compound	Melting Point (°C)	Boiling Point (°C)
NaCl	801	1413
MgO	2852	3600
CaF₂	1418	2533

Exam Tip: When explaining high melting points, always refer to the "strong electrostatic forces of attraction between oppositely charged ions" — not just "strong bonds." This is the key phrase the examiner is looking for.

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2. Brittleness

Ionic compounds are hard but brittle — they shatter when hit with force.

Explanation:

• When a force is applied, layers of ions in the lattice slide over each other.
• Ions of the same charge end up next to each other.
• The repulsion between like charges causes the lattice to shatter.

graph LR
    A["Force applied"] --> B["Layers slide"]
    B --> C["Like charges<br/>align: + next to +<br/>and − next to −"]
    C --> D["Repulsion →<br/>lattice shatters"]

    style A fill:#2c3e50,color:#fff
    style B fill:#2980b9,color:#fff
    style C fill:#e67e22,color:#fff
    style D fill:#c0392b,color:#fff

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3. Electrical Conductivity

Ionic compounds do not conduct electricity when solid but do conduct when molten or dissolved in water.

State	Conducts?	Explanation
Solid	No	Ions are held in fixed positions in the lattice and cannot move to carry a charge
Molten (liquid)	Yes	The lattice breaks down and ions are free to move and carry a charge
Dissolved in water	Yes	The lattice breaks apart and ions are free to move through the solution

Exam Tip: When asked why ionic compounds conduct when molten or dissolved, you must say the "ions are free to move and carry a charge." Saying "electrons are free to move" is incorrect for ionic compounds and will lose marks.

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4. Solubility

Many ionic compounds are soluble in water but insoluble in non-polar solvents (such as hexane).

• Water molecules are polar — they have a slight positive end and a slight negative end.
• The polar water molecules can surround and separate the ions from the lattice, dissolving the compound.

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Comparing Ionic Compounds

Compound	Ions	Charges	Relative Melting Point
NaCl	Na⁺, Cl⁻	+1, −1	Lower
MgO	Mg²⁺, O²⁻	+2, −2	Higher

Magnesium oxide has a much higher melting point than sodium chloride because the charges on the ions are greater (+2 and −2 vs +1 and −1), so the electrostatic forces of attraction are stronger.

Exam Tip: When comparing melting points of ionic compounds, always relate the difference to the size of the charges on the ions and the strength of the electrostatic forces.

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Summary

• Ionic compounds form a giant ionic lattice — a regular arrangement of alternating positive and negative ions.
• The ions are held by strong electrostatic forces of attraction acting in all directions.
• Ionic compounds have high melting and boiling points because a large amount of energy is needed to overcome these strong forces.
• They are brittle because displacing layers brings like charges together, causing repulsion and shattering.
• They do not conduct electricity as solids (ions are fixed) but do conduct when molten or dissolved (ions are free to move).
• Compounds with higher ionic charges have stronger forces and therefore higher melting points.

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Worked Examples — Predicting Properties from Structure

Example 1 — Comparing NaCl and MgO melting points

Both are giant ionic lattices, but magnesium oxide melts at 2852 °C while sodium chloride melts at only 801 °C. The only structural difference is the size of the ionic charges: Mg²⁺ and O²⁻ versus Na⁺ and Cl⁻. The electrostatic attraction between +2 and −2 ions is significantly greater than between +1 and −1 ions, so more energy is required to overcome the ionic bonds in MgO. This is an application of Coulomb's attraction — the force between two charges depends directly on the product of the charges.

Example 2 — Explaining why solid NaCl cannot conduct

Imagine a crystal of sodium chloride. Each Na⁺ is locked in position by six surrounding Cl⁻ ions (and vice versa). There are no free-moving charged particles. Electrons are held within the outer shell of each chloride ion and are not delocalised. Because no charged particle can drift through the lattice, no current flows. The moment the lattice is melted or dissolved, the regular arrangement collapses, the ions separate, and they are free to move towards oppositely charged electrodes — a current now flows.