3.2 Ionic Bonding
Candidates should be able to:
- Define ionic bonding.
- Describe ionic bonding, using the examples of sodium chloride, magnesium chloride and calcium fluoride, including the use of ‘dot-and-cross’ diagrams.
1 What Is Ionic Bonding? ▾
An ionic bond is the strong electrostatic attraction between positively charged ions (cations) and negatively charged ions (anions), held together in a giant crystal lattice.
Think of it like this: opposites attract! Positive and negative ions stick together like tiny magnets, packed into a repeating 3D pattern.
Cations (positive ions) form when an atom loses one or more electrons. Metal atoms usually lose electrons to form cations.
Anions (negative ions) form when an atom gains one or more electrons. Non-metal atoms usually gain electrons to form anions.
2 Formation of Ionic Bonds — Worked Examples ▾
Ionic bonds form between atoms that need to lose electrons to reach the nearest noble gas electronic configuration, and atoms that need to gain electrons to do the same. The electron(s) are transferred from one atom to the other.
Sodium Chloride, NaCl
An electron is transferred from the 3s orbital of sodium to the 3p orbital of chlorine. Both resulting species end up with the electron configuration of the nearest noble gas, and the ions are held together in a crystal lattice by electrostatic attraction.
Magnesium Chloride, MgCl₂
Magnesium (2,8,2) must lose two electrons to reach a stable configuration, so it reacts with two chlorine atoms, each of which gains one electron.
Calcium Fluoride, CaF₂
3 Physical Properties of Ionic Compounds ▾
| Property | Observation & Explanation |
|---|---|
| Melting point | Very high. A large amount of energy must be put in to overcome the strong electrostatic attractions and separate the ions. |
| Strength | Very brittle. Any dislocation causes layers of ions to shift, bringing similar charges adjacent to one another. The resulting repulsion splits the crystal. |
| Electrical conductivity | Do not conduct when solid — the ions are held strongly in a fixed lattice. Do conduct when molten or in aqueous solution, since the ions become mobile and can carry charge. |
| Solubility | Insoluble in non-polar solvents, but soluble in water. Water is a polar solvent that stabilises the separated ions — the energy needed to overcome the electrostatic attraction is compensated by the stability gained as ions become surrounded by polar water molecules. |
- Describe the ionic bond as the electrostatic attraction between oppositely charged ions.
- Include square brackets and the correct charge on every ion in a dot-and-cross diagram.
- Use dots and crosses to show which atom each electron originally came from.
- Check the ion ratio balances overall charge (e.g. one Mg²⁺ to two Cl⁻).
- Quote electron configurations before and after transfer, e.g. 2,8,1 → 2,8.
- Don’t say the ionic bond is the transfer of electrons — that only explains how the ions form.
- Don’t forget the square brackets and charge when drawing an ion.
- Don’t claim ionic compounds never conduct electricity — they do when molten or aqueous.
- Don’t confuse cations (positive, from metals) with anions (negative, from non-metals).
- Ionic bond = electrostatic attraction between positive and negative ions in a lattice.
- Metals lose electrons → positive ions (cations); non-metals gain electrons → negative ions (anions).
- Atoms transfer electrons to reach the nearest noble gas electron configuration.
- Key properties: high melting point, brittle, conducts only when molten or in solution, soluble in water.