Ionic Bonding

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.

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What Is Ionic Bonding?

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.

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How Ions Form

Metal Atoms Form Positive Ions (Cations)

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 Form Negative Ions (Anions)

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.

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The Process of Ionic Bonding: Sodium Chloride (NaCl)

1. A sodium atom (Na) has the electron configuration 2, 8, 1 — it has 1 electron in its outer shell.
2. A chlorine atom (Cl) has the electron configuration 2, 8, 7 — it needs 1 more electron to fill its outer shell.
3. The sodium atom transfers its one outer electron to the chlorine atom.
4. Sodium becomes Na⁺ (electron configuration 2, 8).
5. Chlorine becomes Cl⁻ (electron configuration 2, 8, 8).
6. The oppositely charged ions are attracted by a strong electrostatic force — this is the ionic bond.

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

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The Process of Ionic Bonding: Magnesium Oxide (MgO)

1. A magnesium atom (Mg) has the electron configuration 2, 8, 2 — it has 2 electrons in its outer shell.
2. An oxygen atom (O) has the electron configuration 2, 6 — it needs 2 more electrons.
3. Magnesium transfers both outer electrons to the oxygen atom.
4. Magnesium becomes Mg²⁺ (electron configuration 2, 8).
5. Oxygen becomes O²⁻ (electron configuration 2, 8).

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Compounds Requiring Multiple Atoms: MgCl₂ and Na₂O

When the number of electrons lost does not match the number gained by a single atom, multiple atoms are required.

Magnesium chloride (MgCl₂):

• Magnesium loses 2 electrons, but chlorine can only gain 1.
• One magnesium atom transfers one electron to each of two chlorine atoms.
• Mg²⁺ + 2Cl⁻ → MgCl₂

Sodium oxide (Na₂O):

• Oxygen needs to gain 2 electrons, but sodium can only lose 1.
• Two sodium atoms each transfer one electron to one oxygen atom.
• 2Na⁺ + O²⁻ → Na₂O

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Dot-and-Cross Diagrams

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.

Rules for Drawing Dot-and-Cross Diagrams

1. Show only the outer shell electrons (unless the question specifies otherwise).
2. Use dots for electrons from one atom and crosses for the other.
3. Show the transferred electron(s) in the outer shell of the receiving ion.
4. Draw square brackets around each ion.
5. Write the charge outside the brackets (top right).

Key Compounds You Must Be Able to Draw

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.

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Common Mistakes

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

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Summary

• Ionic bonding is the strong electrostatic force of attraction between oppositely charged ions.
• It occurs between metals (which lose electrons to form positive ions) and non-metals (which gain electrons to form negative ions).
• Both ions achieve a full outer shell — the electron configuration of a noble gas.
• The charge on an ion can be predicted from the group number in the periodic table.
• Dot-and-cross diagrams show the transfer of electrons, using square brackets and charges for each ion.
• The formula of an ionic compound balances so that the total positive and negative charges equal zero.