Ionic bonding and ionic compounds – GCSE Combined Science Chemistry

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Table salt is made of sodium and chlorine. On their own, sodium is a very reactive metal and chlorine is a poisonous gas. Joined together, they make something safe to put on chips. Ionic bonding explains how.

This matters because many compounds you meet are ionic, including table salt and limestone. Knowing how the bonding works lets you predict what a compound is like.

By the end you will be able to say how ions form and draw them. You will describe the structure of an ionic compound. You will work out formulae and explain why ionic compounds melt only at high temperatures.

Ions and ionic bonding

An ion is an atom, or a group of atoms, with an electric charge. Atoms form ions by losing or gaining electrons. This gives them a full outer shell, like a noble gas.

Ionic bonding happens between a metal and a non-metal. The metal loses electrons and becomes a positive ion. The non-metal gains them and becomes a negative ion. Opposite charges attract strongly.

Group 1 forms 1+ ions and Group 2 forms 2+ ions. Group 6 forms 2− ions and Group 7 forms 1− ions.

Dot and cross diagrams

A dot and cross diagram shows the electrons moving. Draw the electrons of one atom as dots and the other as crosses. Think of passing a ball from one player to another.

Sodium has one outer electron and chlorine has seven. Sodium passes its electron to chlorine. Sodium (2,8,1) becomes Na⁺ (2,8). Chlorine (2,8,7) becomes Cl⁻ (2,8,8).

Draw each ion in square brackets with its charge at the top right. In magnesium oxide, the magnesium ion is 2+ and the oxide ion is 2−.

Giant ionic lattices

An ionic compound is a giant lattice. This is a regular, repeating pattern of positive and negative ions. Each ion is held by strong electrostatic forces of attraction. These forces act in all directions.

There are no separate molecules. The formula shows the simplest ratio of ions. In sodium chloride there is one Na⁺ for each Cl⁻, so the formula is NaCl.

Picture a tray of oranges and apples packed in a neat grid, with each one touching its neighbours.

Properties of ionic compounds

Ionic compounds have high melting and boiling points. A lot of energy is needed to overcome the strong forces between ions. They are solids at room temperature.

They do not conduct when solid, because the ions are fixed in place. They do conduct when molten or dissolved in water, because the ions are free to move and carry the charge.

Higher tier only. Bigger charges mean stronger attraction. Magnesium oxide, with Mg²⁺ and O²⁻, has a much higher melting point than sodium chloride.

Working out formulae

The total positive charge must equal the total negative charge. Magnesium chloride has Mg²⁺ and Cl⁻. One Mg²⁺ needs two Cl⁻ to balance, so the formula is MgCl₂. Think of balancing a see-saw.

For aluminium oxide, the ions are Al³⁺ and O²⁻. The lowest common multiple of 3 and 2 is 6. That needs two Al³⁺ (6+) and three O²⁻ (6−), so the formula is Al₂O₃.

Use brackets for groups of atoms, such as Ca(OH)₂.

Naming follows rules. A compound of a metal with one non-metal ends in -ide, for example sodium chloride and magnesium oxide. A compound with an ion that contains oxygen as well ends in -ate, for example sulfate (SO₄²⁻), nitrate (NO₃⁻) and carbonate (CO₃²⁻). Hydroxide (OH⁻) is an exception to this pattern.

Half equations

Higher tier only. A half equation shows one ion forming. Sodium loses an electron: Na → Na⁺ + e⁻. Chlorine gains electrons: Cl₂ + 2e⁻ → 2Cl⁻. The charges must balance on both sides.

A metal loses electrons, so the electrons are on the right. Magnesium loses two: Mg → Mg²⁺ + 2e⁻. A non-metal gains electrons, so they are on the left.

The overall equation is 2Na + Cl₂ → 2NaCl. Two sodium atoms each lose one electron. One chlorine molecule gains those two electrons.

Recap

Metals lose electrons to form positive ions. Non-metals gain electrons to form negative ions.

Ionic compounds are giant lattices held by strong electrostatic forces. They have high melting points and conduct only when molten or dissolved.

The total positive charge equals the total negative charge in the formula.

A half equation shows electrons lost or gained.