2.1.2 Ionic Bonding — AQA GCSE Combined Science Trilogy 8464 - Chemistry Revision Notes & Topical Questions

Section 1 of 7Ionic Bonding

Essential knowledge

Formation of ions

Formation of ions atoms lose or gain electrons to reach the stable electron arrangement of a noble gas. Metals, with 1–3 outer electrons, lose them to form positive ions; non-metals, with 5–7 outer electrons, gain electrons to form negative ions.

Oxidation

Loss of electrons is oxidation and gain of electrons is reduction, e.g. Mg → Mg²⁺ + 2e⁻ and Cl₂ + 2e⁻ → 2Cl⁻.

Common ions

Common ions learn charges from group numbers, plus transition metal ions shown by Roman numerals (iron(II) Fe²⁺, iron(III) Fe³⁺, copper(II) Cu²⁺), silver Ag⁺, zinc Zn²⁺,

and compound ions: ammonium NH₄⁺, hydroxide OH⁻, nitrate NO₃⁻, carbonate CO₃²⁻, sulfate SO₄²⁻.

Finding formulae

Finding formulae choose numbers of each ion so charges cancel.

Calcium chlorideCa²⁺ needs two Cl⁻ → CaCl₂.
Ammonium sulfatetwo NH₄⁺ to one SO₄²⁻ → (NH₄)₂SO₄.
Endings-ide = element only, -ate = contains oxygen.

Dot and cross diagrams

Dot and cross diagrams show electron transfer: use dots for one atom's electrons and crosses for the other's. Final ions are drawn in square brackets with the charge outside; often only outer shells are shown. For MgO, two electrons move from Mg to O.

Structure

Structure ionic compounds form a giant lattice, where each ion is surrounded by ions of the opposite charge, and attractions act in all directions throughout the structure.

Properties

Properties high melting and boiling points because lots of energy is needed to overcome the many strong electrostatic attractions. They conduct electricity only when molten or dissolved, because then the ions can move. Solids are hard but brittle.

Ions and subatomic particles

Ions and subatomic particles a cation has more protons than electrons; an anion has more electrons than protons. The size of the charge equals the number of electrons lost or gained, e.g. an aluminium atom losing 3 electrons forms Al³⁺.

Predicting charges

Predicting charges atoms with 1–3 outer electrons usually lose them; atoms with 5–7 outer electrons usually gain enough to reach 8. Group 4 elements rarely form simple ions.

More common ions

More common ions also learn lead(II) Pb²⁺ and hydrogen H⁺. Like ammonium NH₄⁺, H⁺ is a positive ion that does not come from a metal.

Swap-and-drop

Swap-and-drop to find a formula quickly, use the size of each ion's charge as the number of the other ion, then simplify. Al³⁺ and O²⁻ give Al₂O₃; Mg²⁺ and O²⁻ give Mg₂O₂, which simplifies to MgO.

Dot and cross

Dot and cross other groups

in lithium nitride, three lithium atoms (2.1) each give one electron to one nitrogen atom (2.5). This forms three Li⁺ ions (2) and one N³⁻ ion (2.8), so the formula is Li₃N.

Conduction

Conduction ions, not electrons

molten or dissolved ionic compounds conduct because their ions move and carry charge. Saying that electrons move through the compound is a common mistake that loses marks.

Section 2 of 7Process & sequence

1

Write the electronic configuration of each atom (e.g. Na 2.8.1, Cl 2.8.7).

2

The metal atom transfers its outer electron(s) to the non-metal atom.

3

The metal becomes a positive ion with a full outer shell (Na⁺ 2.8).

4

The non-metal becomes a negative ion with a full outer shell (Cl⁻ 2.8.8).

5

Draw each ion in square brackets with its charge at the top right.

6

The oppositely charged ions attract, forming an ionic bond in a giant lattice.

Forming sodium chloridewrite these steps in this order; each step is a marking point.

1

Na 2.8.1 and Cl 2.8.7

2

Na transfers one electron

3

Na⁺ 2.8 formed

4

Cl⁻ 2.8.8 formed

5

Opposite charges attract

6

Giant ionic lattice

Section 3 of 7Compare

Common ions
Ion	Formula	Ion	Formula
Sodium	Na⁺	Chloride	Cl⁻
Magnesium	Mg²⁺	Oxide	O²⁻
Aluminium	Al³⁺	Nitride	N³⁻
Ammonium	NH₄⁺	Hydroxide	OH⁻
Copper(II)	Cu²⁺	Sulfate / Carbonate / Nitrate	SO₄²⁻ / CO₃²⁻ / NO₃⁻
Iron(III)	Fe³⁺	Sulfide	S²⁻

Properties of ionic compounds
Property	Reason
High melting and boiling points	Much energy needed to overcome strong electrostatic forces in the lattice
Do not conduct when solid	Ions are fixed in position
Conduct when molten or dissolved	Ions are free to move and carry charge
Often soluble in water	Water molecules attract and separate the ions

Section 4 of 7Worked examples

1Work out the formula of iron(III) sulfate.

Fe³⁺ and SO₄²⁻. Lowest common multiple of 3 and 2 is 6: 2 Fe³⁺ = +6, 3 SO₄²⁻ = −6. Formula: Fe₂(SO₄)₃.

2

Explain why magnesium oxide has a much higher melting point than sodium chloride.

Both are giant ionic lattices, but MgO contains Mg²⁺ and O²⁻ ions with double the charges of Na⁺ and Cl⁻.
- The electrostatic attractions are stronger, so more energy is needed to overcome them.

Section 5 of 7Key terms

Section 6 of 7Command terms

Command terms

Give / State / Name

Recall a fact; one idea per mark.

Define

Give the exact meaning using the specification keywords.

Describe

Give an account of what happens / the pattern in data, quoting values; no reasons needed.

Explain

Give reasons with linked statements (cause → effect). Each linked step is usually a separate marking point.

Suggest

Use knowledge in a new context; more than one answer may be accepted.

Compare

Similarities and differences; each point must refer to both items.

Calculate

Show working; include units; check significant figures. Method marks are given even if the final answer is wrong.

Discuss

Explore the issue from different angles and link the points.

Evaluate / Assess

Weigh strengths and weaknesses and reach a supported judgement.

Deduce

Draw a conclusion from the information given.

Justify

Support a statement/choice with evidence.

Section 7 of 7Exam technique

Keywords the mark scheme looks for in this lesson

Ion · Cation · Anion · Ionic bond · Giant ionic lattice · Compound ion · Dot and cross diagram · Noble gas configuration · Electrostatic attraction

Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.

How to get full marks (examiner guidance)

Use specification keywords exactly (e.g. electronegativity, nucleophile, dynamic equilibrium, activation energy).

Link ideas

Link ideas for “explain” each marking point is usually a cause → consequence step; unlinked facts often score nothing.

Use the information in the question

“discuss/evaluate” answers must use the data provided and match the number of marks.

Convert units carefully (mg → g, cm³ → dm³) and show working for calculations.

Avoid vague words

“it”, “stuff”, “things”, “goes up”, “energy is made”. Name the substance and the process.

Do not extrapolate a graph line beyond the data or to zero unless asked.

Six-mark extended responses are marked on content AND a logical, linked line of reasoning.