Lattice – IGCSE Chemistry Definition

A lattice is a regular, repeating three-dimensional arrangement of particles in a crystalline solid. The particles can be ions (in ionic compounds), atoms (in giant covalent structures and metals), or molecules (in molecular crystals). The term is important at IGCSE because it explains why certain substances have characteristic physical properties such as high melting points, hardness, and brittleness.

Ionic lattice

In an ionic compound such as sodium chloride, the lattice consists of alternating Na⁺ and Cl⁻ ions arranged in a regular cubic pattern. Each Na⁺ ion is surrounded by Cl⁻ ions, and each Cl⁻ ion is surrounded by Na⁺ ions. The strong electrostatic attractions between the oppositely charged ions hold the structure together in all directions. This is why ionic compounds have high melting points, are hard, and are brittle (if the layers shift, like charges end up next to each other and repel, causing the crystal to shatter).

Giant covalent lattice

In diamond, every carbon atom is covalently bonded to four other carbon atoms in a tetrahedral arrangement that repeats throughout the entire structure. In silicon dioxide, each silicon atom is bonded to four oxygen atoms and each oxygen atom is bonded to two silicon atoms. These giant networks of strong covalent bonds produce very high melting points.

Metallic lattice

In metals, the lattice consists of positive metal ions arranged in regular layers with delocalised electrons moving between them. The electrostatic attraction between the ions and the electron sea gives metals their strength, conductivity, and malleability.

Exam context

Examiners test whether candidates can link the type of lattice to physical properties. A common question provides melting point data and asks candidates to identify the type of structure. High melting point with electrical conductivity when molten indicates an ionic lattice. Very high melting point with no conductivity indicates a giant covalent lattice.

Worked exam question

Sodium chloride has a melting point of 801 degC. Explain this in terms of its structure. (2 marks)

Sodium chloride has a giant ionic lattice [1]. There are strong electrostatic forces of attraction between the oppositely charged Na⁺ and Cl⁻ ions throughout the structure, and a large amount of energy is needed to overcome these forces [1].

Common mistakes

Candidates often say ionic compounds have “strong ionic bonds” without specifying what these forces are between. The mark scheme requires reference to electrostatic attraction between oppositely charged ions. Another error is saying “molecules” when describing an ionic lattice --- ionic compounds do not contain molecules; they contain ions.