Giant Ionic Structure – IGCSE Chemistry Definition and Key Facts

Giant ionic structures are central to the Cambridge 0620 syllabus topic on bonding and structure. Understanding this structure is essential for explaining the physical properties of ionic compounds, a question type that appears on almost every Paper 4.

The 0620 definition

A giant ionic structure is a three-dimensional regular arrangement (lattice) of positive and negative ions held together by strong electrostatic forces of attraction between oppositely charged ions. The structure extends in all directions and contains billions of ions — there are no individual molecules.

The sodium chloride lattice

Sodium chloride (NaCl) is the standard example. In the lattice:

• Each Na⁺ ion is surrounded by 6 Cl⁻ ions
• Each Cl⁻ ion is surrounded by 6 Na⁺ ions
• Ions alternate in a regular cubic pattern
• The ratio of Na⁺ to Cl⁻ is 1:1, matching the formula NaCl

The formula NaCl represents the simplest ratio of ions, not a molecule. It is incorrect to refer to “a molecule of NaCl.”

Properties explained by the structure

High melting and boiling points

The strong electrostatic forces between billions of oppositely charged ions require a large amount of energy to overcome. NaCl melts at 801 °C and boils at 1413 °C.

Exam tip: always say “strong electrostatic forces between oppositely charged ions” — not just “strong bonds.”

Hard but brittle

The lattice is rigid because the ions are held firmly in position by the electrostatic attractions. However, if a force is applied that shifts a layer of ions, ions of the same charge are brought next to each other. The repulsion between like charges causes the lattice to shatter. This is why ionic crystals are brittle, not malleable or ductile.

Electrical conductivity

State	Conducts?	Reason
Solid	No	Ions are fixed in position and cannot move
Molten (liquid)	Yes	Ions are free to move and carry charge
Aqueous (dissolved)	Yes	Ions are free to move in solution

This property directly links to electrolysis — ionic compounds must be molten or dissolved to conduct and undergo electrolysis.

Solubility in water

Many ionic compounds dissolve in water. Water molecules, which are polar, attract the ions on the surface of the lattice and pull them into solution. This process is called hydration of ions. Not all ionic compounds are soluble — the solubility rules must be learned.

Giant ionic structure vs other structures

Structure type	Example	Melting point	Conducts when solid?	Conducts when liquid?
Giant ionic	NaCl	High	No	Yes
Simple molecular	H₂O, CO₂	Low	No	No
Giant covalent	Diamond, SiO₂	Very high	No (except graphite)	No
Metallic	Cu, Fe	High	Yes	Yes

Worked exam question

Sodium chloride has a high melting point and conducts electricity when dissolved in water but not when solid. Explain these properties in terms of its structure and bonding. [4]

NaCl has a giant ionic structure/lattice [1]. Strong electrostatic forces between oppositely charged Na⁺ and Cl⁻ ions require a lot of energy to overcome, so the melting point is high [1]. In the solid, ions are held in fixed positions and cannot move, so it does not conduct [1]. When dissolved, the ions are free to move through the solution and carry the electrical charge [1].

Common exam mistakes

1. Saying ionic compounds have “strong bonds between molecules” — there are no molecules in an ionic compound. Use “strong electrostatic forces between ions.”
2. Saying a solid ionic compound cannot conduct because “there are no electrons” — the correct reason is that the ions cannot move.
3. Forgetting the word “oppositely charged” when describing the attraction between ions — examiners look for this phrase.