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:

$\text{Na} \quad (2, 8, 1) \xrightarrow{\text{loses 1 e}^-} \quad \text{Na}^+ \quad (2, 8)$Na(2,8,1)loses 1 e−​Na+(2,8)

$\text{Mg} \quad (2, 8, 2) \xrightarrow{\text{loses 2 e}^-} \quad \text{Mg}^{2+} \quad (2, 8)$Mg(2,8,2)loses 2 e−​Mg2+(2,8)

$\text{Al} \quad (2, 8, 3) \xrightarrow{\text{loses 3 e}^-} \quad \text{Al}^{3+} \quad (2, 8)$Al(2,8,3)loses 3 e−​Al3+(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.

$\text{Cl} \quad (2, 8, 7) \xrightarrow{\text{gains 1 e}^-} \quad \text{Cl}^- \quad (2, 8, 8)$Cl(2,8,7)gains 1 e−​Cl−(2,8,8)

$\text{O} \quad (2, 6) \xrightarrow{\text{gains 2 e}^-} \quad \text{O}^{2-} \quad (2, 8)$O(2,6)gains 2 e−​O2−(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.

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Worked Example 1 — Predicting the Formula of Calcium Fluoride

To predict the formula of an ionic compound from the constituent elements, use charges derived from the periodic table group.

1. Calcium is in Group 2 — it loses 2 outer-shell electrons to form Ca²⁺.
2. Fluorine is in Group 7 — it gains 1 electron to form F⁻.
3. The overall charge on the compound must be zero.
4. One Ca²⁺ requires two F⁻ to balance: (+2) + 2 × (−1) = 0.
5. Formula: CaF₂.

This technique, sometimes called the "swap-and-drop" or "cross-over" method, is reliable but must be supported by an understanding of why the charges arise. The underlying principle is electrostatic attraction between oppositely charged ions, and the charges themselves arise because each ion now has the electron configuration of the nearest noble gas.

Worked Example 2 — Aluminium Oxide from Dot-and-Cross Reasoning

Aluminium has electron configuration 2, 8, 3 and oxygen has 2, 6.

• Each Al atom loses 3 electrons to form Al³⁺ (2, 8).
• Each O atom gains 2 electrons to form O²⁻ (2, 8).
• For charges to balance we need the lowest common multiple of 3 and 2, which is 6.
• Therefore 2 × Al³⁺ transfers 6 electrons in total, and 3 × O²⁻ receives those 6 electrons.
• Formula: Al₂O₃.

When drawing the dot-and-cross diagram, write two aluminium ions in square brackets with ³⁺ outside, and three oxide ions in square brackets with ²⁻ outside. Show that the outer shell of each ion is now a full octet.

Worked Example 3 — Predicting Properties of a New Ionic Compound

Suppose a question gives you lithium sulfide and asks you to predict the melting point relative to sodium chloride.

1. Li is Group 1 → Li⁺; S is Group 6 → S²⁻; formula Li₂S.
2. Li⁺ is a smaller cation than Na⁺, so the electrostatic attraction is stronger for a given anion.
3. S²⁻ has a greater charge than Cl⁻, further increasing the attractive force.
4. Prediction: Li₂S has a higher melting point than NaCl. (Actual value: ~938 °C vs 801 °C — consistent.)

This kind of reasoning earns marks in extended-response questions because it links structure (giant ionic lattice), bonding (electrostatic attraction) and properties (high melting point) in a single logical chain.

Comparison Table — Ion Formation Across the Periodic Table

Group	Outer electrons	Electrons lost/gained	Ion formed	Example
1	1	Lose 1	1+	Na⁺, K⁺, Li⁺
2	2	Lose 2	2+	Mg²⁺, Ca²⁺
3	3	Lose 3	3+	Al³⁺
5	5	Gain 3	3−	N³⁻
6	6	Gain 2	2−	O²⁻, S²⁻
7	7	Gain 1	1−	F⁻, Cl⁻, Br⁻
0	8 (or 2 for He)	Neither	None	Ne, Ar (unreactive)

graph TD
    A["Identify the two elements"] --> B{"Metal + Non-metal?"}
    B -->|"Yes"| C["Ionic bond forms"]
    B -->|"No"| Z["Not ionic"]
    C --> D["Find group of metal<br/>→ charge of cation"]
    C --> E["Find group of non-metal<br/>→ charge of anion"]
    D --> F["Balance charges<br/>to get formula"]
    E --> F
    F --> G["Draw dot-and-cross<br/>with square brackets<br/>and charge labels"]

    style A fill:#2c3e50,color:#fff
    style C fill:#27ae60,color:#fff
    style F fill:#f39c12,color:#fff
    style G fill:#8e44ad,color:#fff

Common-Mistake Callouts

Callout — "transferred, not shared": In a dot-and-cross diagram for an ionic bond, the transferred electron is drawn in the outer shell of the receiving ion. It is shown using the symbol of the donor atom (typically a cross if the donor uses crosses, or vice versa), even though it is now physically part of the anion. Never draw a shared pair between two brackets — that would imply covalent bond character.

Callout — charge magnitude matters: Higher magnitudes of ionic charge (2+ with 2−, or 3+ with 2−) create much stronger electrostatic attraction than 1+ with 1−. This is why MgO melts at 2852 °C but NaCl melts at 801 °C. Exam markers reward candidates who mention "higher charges" and "stronger attraction" explicitly.

Callout — spectator electrons: Some students try to show every electron in both atoms. Only the outer shell is required for AQA 8464 dot-and-cross diagrams unless the question states otherwise. Drawing inner shells is not penalised but wastes time.

Answering at different grade levels

Grade band	How a response to "Describe the bonding in sodium chloride" might read
Grades 1–3	"Sodium and chlorine join. Sodium gives an electron to chlorine. They form NaCl." This mentions transfer but does not name the bond or the force.
Grades 4–5	"Sodium (a metal) transfers its outer electron to chlorine (a non-metal). Sodium becomes Na⁺ and chlorine becomes Cl⁻. The ionic bond is formed between the two ions." Uses precise term ionic bond and correct electron transfer.
Grades 6–7	"An electron is transferred from the outer shell of the sodium atom to the outer shell of the chlorine atom, forming Na⁺ (2,8) and Cl⁻ (2,8,8). Both ions now have a full outer shell, the same electron configuration as a noble gas. The ionic bond is the strong electrostatic attraction between the oppositely charged ions." Uses electron configurations and the defining word electrostatic.
Grades 8–9	"Ionic bonding is the strong electrostatic attraction between oppositely charged ions held in a giant ionic lattice. In NaCl, each Na⁺ is surrounded by six Cl⁻ and vice versa, so the attractive forces act in all directions throughout the lattice. The bond is non-directional; the formula NaCl represents the simplest whole-number ratio of ions rather than a discrete molecule." Includes structure, geometry, and the distinction between formula unit and molecule.

AQA alignment: This content is aligned with AQA GCSE Combined Science: Trilogy (8464) specification section 5.2 Bonding, structure and the properties of matter — specifically 5.2.1 Chemical bonds, ionic, covalent and metallic, and 5.2.2 How bonding and structure are related to the properties of substances. Assessed on Chemistry Paper 1.