Ionic compounds — AQA GCSE Chemistry

An ionic compound is not made of separate molecules. Instead, it forms a giant structure, also called a giant ionic lattice, in which huge numbers of positive and negative ions are packed together in a regular repeating pattern. Each ion is surrounded by ions of opposite charge. For example, in sodium chloride, each Na⁺ ion is surrounded by six Cl⁻ ions, and each Cl⁻ ion is surrounded by six Na⁺ ions. The structure extends in three dimensions, so there is no fixed number of ions in a single piece of the compound. The formula, such as NaCl, shows the simplest ratio of ions, not a molecule. This giant arrangement explains the high melting points of ionic compounds because many strong attractions must be overcome.

In an ionic compound, positive and negative ions attract each other because opposite charges attract. These attractions act in all directions throughout the giant lattice and are called electrostatic forces of attraction. They are strong, so a large amount of energy is needed to overcome them. This is why ionic compounds have high melting points and high boiling points. For example, sodium chloride melts at about 801 °C because the Na⁺ and Cl⁻ ions are held in a strong three-dimensional network. When an ionic compound melts or dissolves, the lattice breaks down and the ions become free to move, which allows the molten or dissolved compound to conduct electricity.

Ionic bonding is the strong electrostatic attraction between oppositely charged ions. In a giant ionic lattice, each ion is surrounded by ions of opposite charge, and the attractive forces act in all directions rather than along a single line. For example, in sodium chloride, each Na⁺ ion is attracted to six Cl⁻ ions arranged octahedrally, and each Cl⁻ ion is attracted to six Na⁺ ions. There is no discrete molecule; the lattice is a continuous three-dimensional array. The term 'ionic bonding' therefore describes the net effect of many directional attractions, not a shared pair of electrons. This explains why ionic compounds have high melting points and are often soluble in water.

Sodium chloride has a giant ionic lattice that can be represented in several ways. A dot and cross diagram shows the transfer of electrons from sodium to chlorine, forming Na⁺ and Cl⁻ ions, but it usually shows only one formula unit. A ball and stick model uses spheres for ions and sticks for bonds, making the three-dimensional arrangement clear but implying that ions are joined by directional bonds. A two-dimensional diagram shows a flat cross-section of the lattice, which is easy to draw but does not show the full three-dimensional arrangement. A three-dimensional model shows the true arrangement of ions in space, with each Na⁺ surrounded by six Cl⁻ ions and vice versa. Each representation has limitations, so chemists choose the one that best communicates the feature they want to show.

This statement combines three skills. First, you must recognise an ionic compound from a diagram: look for a giant lattice of oppositely charged ions, often shown as spheres or with charges. Second, you must describe the limitations of dot and cross, ball and stick, and two- and three-dimensional diagrams. Dot and cross diagrams show electron transfer but usually only one formula unit; ball and stick models show arrangement but imply directional bonds; two-dimensional diagrams are flat and miss the three-dimensional lattice; three-dimensional models are most accurate but can be complex. Third, you must work out the empirical formula from a model by counting the ratio of cations to anions. For example, if a diagram shows twice as many Cl⁻ ions as Mg²⁺ ions, the empirical formula is MgCl₂ because the ratio is 1:2.

You need to know the structure of sodium chloride in detail. It is a giant ionic lattice with Na⁺ and Cl⁻ ions arranged in a regular three-dimensional pattern. Each Na⁺ ion is surrounded by six Cl⁻ ions, and each Cl⁻ ion is surrounded by six Na⁺ ions. The empirical formula is NaCl because the ratio of Na⁺ to Cl⁻ is 1:1. You should be able to describe this structure, draw or interpret diagrams of it, and explain its properties, such as its high melting point, using the lattice model. You do not need to recall the structures of other ionic compounds, such as magnesium oxide or calcium chloride, although you may be asked to apply the same principles to them if the information is provided.