The structure of sodium chloride can be represented in the following forms:

The idea

Sodium chloride is a giant ionic lattice, not a collection of separate NaCl molecules. Its structure can be represented in several ways, each showing different information. A dot-and-cross diagram shows electron transfer from sodium to chlorine, giving Na⁺ and Cl⁻ with full outer shells. A ball-and-stick model shows the regular three-dimensional arrangement of ions, with each Na⁺ surrounded by six Cl⁻ ions and each Cl⁻ by six Na⁺ ions. A space-filling model shows how the ions pack together and why the lattice is dense. A two-dimensional diagram or a formula unit such as NaCl shows the simplest ratio of ions, 1:1, not a molecule. Whichever representation is used, the key ideas are strong electrostatic attraction between oppositely charged ions, a giant lattice, and a high melting point because many strong bonds must be broken.

What earns the marks

• Sodium atoms lose one electron to form Na⁺ ions and chlorine atoms gain one electron to form Cl⁻ ions, giving full outer electron shells.
• The ions are held in a giant three-dimensional lattice by strong electrostatic attraction between oppositely charged ions.
• Each Na⁺ ion is surrounded by six Cl⁻ ions and each Cl⁻ ion by six Na⁺ ions in the regular lattice arrangement.
• A dot-and-cross diagram represents electron transfer and outer-shell electron configurations, while a ball-and-stick or space-filling model represents the three-dimensional lattice arrangement.
• The formula NaCl represents the simplest ratio of sodium ions to chloride ions, 1:1, and does not represent a discrete molecule.
• The strong electrostatic forces throughout the giant lattice explain the high melting point of sodium chloride.

Common mistakes and how to put them right

• Writing NaCl as a small molecule with a single Na–Cl covalent bond.

  Put it right by describing a giant ionic lattice with many Na⁺ and Cl⁻ ions.

• Drawing a dot-and-cross diagram with shared electrons instead of transferred electrons.

  Put it right by showing Na⁺ with no outer electrons and Cl⁻ with eight outer electrons.

• Stating that sodium chloride conducts electricity when solid.

  Put it right by explaining that ions are fixed in the lattice, so conduction occurs only when molten or dissolved.

Exam tips

• When asked to represent sodium chloride, label the ions and their charges clearly, for example Na⁺ and Cl⁻, and state the 1:1 ratio.
• If a question asks why sodium chloride has a high melting point, link the answer to many strong electrostatic attractions in the giant lattice, not to a single bond.
• Use the representation named in the question; if it asks for a dot-and-cross diagram, do not substitute a ball-and-stick model.