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This lesson covers ionic bonding as required by the AQA GCSE Combined Science Trilogy specification (8464), section 4.2.1. You need to understand how ionic bonds form through the transfer of electrons, predict the charges of ions from their position in the periodic table, and draw dot-and-cross diagrams to represent the formation of ionic compounds.
Ionic bonding is the strong electrostatic force of attraction between oppositely charged ions. It occurs when atoms transfer electrons from one to another, forming positive ions (cations) and negative ions (anions).
Ionic bonds form between metals and non-metals. The metal atom loses electrons while the non-metal atom gains electrons, so that both achieve a full outer shell of electrons — the same electron configuration as a noble gas.
Exam Tip (AQA 8464): The precise definition is: "Ionic bonding is the strong electrostatic force of attraction between oppositely charged ions." Do not say "between a metal and a non-metal" — that describes when ionic bonding occurs, not what it is.
Metal atoms have a small number of electrons in their outer shell (typically 1, 2 or 3). They achieve a full outer shell by losing these outer electrons. Because the atom then has more protons than electrons, it carries an overall positive charge.
The electron configuration of the resulting ion is the same as the nearest noble gas:
$$\text{Na} \quad (2, 8, 1) \xrightarrow{\text{loses 1 e}^-} \quad \text{Na}^+ \quad (2, 8)$$
$$\text{Mg} \quad (2, 8, 2) \xrightarrow{\text{loses 2 e}^-} \quad \text{Mg}^{2+} \quad (2, 8)$$
$$\text{Al} \quad (2, 8, 3) \xrightarrow{\text{loses 3 e}^-} \quad \text{Al}^{3+} \quad (2, 8)$$
| Metal | Group | Electrons Lost | Ion Formed | Electron Configuration |
|---|---|---|---|---|
| Sodium (Na) | 1 | 1 | Na⁺ | 2, 8 (same as neon) |
| Magnesium (Mg) | 2 | 2 | Mg²⁺ | 2, 8 (same as neon) |
| Aluminium (Al) | 3 | 3 | Al³⁺ | 2, 8 (same as neon) |
| Potassium (K) | 1 | 1 | K⁺ | 2, 8, 8 (same as argon) |
| Calcium (Ca) | 2 | 2 | Ca²⁺ | 2, 8, 8 (same as argon) |
Non-metal atoms have outer shells that are close to being full (typically 5, 6 or 7 electrons). They achieve a full outer shell by gaining electrons. Because the atom then has more electrons than protons, it carries an overall negative charge.
$$\text{Cl} \quad (2, 8, 7) \xrightarrow{\text{gains 1 e}^-} \quad \text{Cl}^- \quad (2, 8, 8)$$
$$\text{O} \quad (2, 6) \xrightarrow{\text{gains 2 e}^-} \quad \text{O}^{2-} \quad (2, 8)$$
| Non-Metal | Group | Electrons Gained | Ion Formed | Electron Configuration |
|---|---|---|---|---|
| Fluorine (F) | 7 | 1 | F⁻ | 2, 8 (same as neon) |
| Chlorine (Cl) | 7 | 1 | Cl⁻ | 2, 8, 8 (same as argon) |
| Oxygen (O) | 6 | 2 | O²⁻ | 2, 8 (same as neon) |
| Sulfur (S) | 6 | 2 | S²⁻ | 2, 8, 8 (same as argon) |
| Nitrogen (N) | 5 | 3 | N³⁻ | 2, 8 (same as neon) |
Exam Tip: The charge on an ion is directly related to the group number. Group 1 metals form 1+ ions, Group 2 form 2+ ions, Group 6 non-metals form 2− ions, and Group 7 form 1− ions.
graph LR
A["Na atom<br/>2, 8, 1"] -->|"Loses 1 electron"| B["Na⁺ ion<br/>2, 8"]
C["Cl atom<br/>2, 8, 7"] -->|"Gains 1 electron"| D["Cl⁻ ion<br/>2, 8, 8"]
B -->|"Electrostatic attraction"| E["NaCl<br/>Ionic compound"]
D -->|"Electrostatic attraction"| E
style A fill:#3498db,color:#fff
style B fill:#e74c3c,color:#fff
style C fill:#27ae60,color:#fff
style D fill:#8e44ad,color:#fff
style E fill:#f39c12,color:#fff
When the number of electrons lost does not match the number gained by a single atom, multiple atoms are required.
Magnesium chloride (MgCl₂):
Sodium oxide (Na₂O):
In the AQA 8464 exam, you must draw dot-and-cross diagrams to show ionic bonding. These show the outer shell electrons using dots for one atom and crosses for the other.
| Compound | Electron Transfer | Formula |
|---|---|---|
| Sodium chloride | 1 electron: Na → Cl | NaCl |
| Magnesium oxide | 2 electrons: Mg → O | MgO |
| Magnesium chloride | 1 electron each: Mg → 2Cl | MgCl₂ |
| Sodium oxide | 1 electron each: 2Na → O | Na₂O |
| Calcium chloride | 1 electron each: Ca → 2Cl | CaCl₂ |
Exam Tip: Always include the square brackets and the charge on each ion. Forgetting the charge notation (e.g. [Na]⁺ and [Cl]⁻) will lose you marks even if the electron transfer is shown correctly.
| Mistake | Correction |
|---|---|
| Saying ionic bonding involves "sharing" electrons | Ionic bonding involves the transfer of electrons — sharing is covalent bonding |
| Saying protons are transferred | Only electrons move; the nucleus does not change |
| Forgetting charges on dot-and-cross diagrams | Always include square brackets and the ion charge |
| Writing an incorrect formula | The charges must balance to give an overall charge of zero |
| Confusing atoms and ions | Atoms are neutral; ions are charged particles |