Ionic bonding & formulae

7 practice questions

Ion formation and ionic bonding

Learning goalDistinguish electron transfer from the attraction between ions.

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There are two ideas to keep separate. Electron transfer makes the ions. The attraction between the resulting positive and negative ions is the ionic bonding.

A metal atom loses electrons and becomes a positive ion. A non-metal atom gains electrons and becomes a negative ion. In a giant ionic lattice, each ion attracts surrounding ions of the opposite charge.

In a dot-and-cross diagram, use dots and crosses consistently to show where the electrons came from. Show outer-shell electrons, put each ion in square brackets and write its charge outside. Check both electron totals and the ratio of ions.

Ion formation: Metals lose electrons to form positive ions; non-metals gain electrons to form negative ions.
Ionic bonding: Strong electrostatic attraction between oppositely charged ions in a lattice.

Group	Electron change	Typical ion charge	Example
1	Loses 1	1+	Na⁺
2	Loses 2	2+	Mg²⁺
3	Loses 3	3+	Al³⁺
5	Gains 3	3−	N^3-
6	Gains 2	2−	O^2-
7	Gains 1	1−	Cl^-

EXAM TRAPElectron transfer makes the ions. It is not the definition of the ionic bond.

🧪 TEST IT

Work on paper. Attempt each question before opening its answer. These are original CrackingChem questions, written in AQA style.

Exam-style question 1

Predict the ions formed by potassium, magnesium, nitrogen and sulfur atoms. [4]
Explain why “ionic bonding is the transfer of electrons” is not a correct definition. Give the correct bonding description. [3]

Total: 7 marks

Show answer & marking points

K⁺ [1]; Mg²⁺ [1]; N^3- [1]; S^2- [1].
Electron transfer forms the ions [1]. Ionic bonding is electrostatic attraction [1] between oppositely charged ions in a lattice [1].

✓ DONE WHENYou can predict common ion charges and define ionic bonding precisely.

Ionic formulae and compound ions

Learning goalConstruct neutral ionic formulae using charges and brackets.

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An ionic formula must have no overall charge. Choose the smallest whole-number ratio of ions that makes the positive and negative charges balance.

For example, one 2+ ion needs two 1- ions. Use the charges to find the ratio, then write the subscripts in the formula.

A compound ion stays together as a group. If you need more than one of that ion, put brackets around its formula before adding the subscript. The table below gives common ions to recognise.

Compound ion	Formula
Sulfate	SO₄^2-
Hydroxide	OH^-
Nitrate	NO₃^-
Carbonate	CO₃^2-
Ammonium	NH₄⁺

Worked example

Aluminium sulfate

Construct the formula from Al³⁺ and SO₄^2-.

Worked answer

Two Al³⁺ ions give +6.
Three sulfate ions give −6.
Keep sulfate intact and use brackets: Al₂(SO₄)₃.
Check neutrality and the smallest ratio.

Al₂(SO₄)₃

Ions	Ratio	Formula
Mg²⁺ and Cl^-	1 : 2	MgCl₂
Al³⁺ and O^2-	2 : 3	Al₂O₃
Ca²⁺ and NO₃^-	1 : 2	Ca(NO₃)₂
Ca²⁺ and O^2-	1 : 1	CaO

EXAM TRAP“Drop and swap” can leave an unsimplified formula. Use brackets for repeated compound ions, never alter their internal formulae.

🧪 TEST IT

Work on paper. Attempt each question before opening its answer. These are original CrackingChem questions, written in AQA style.

Exam-style question 2

State the formula and charge of sulfate, hydroxide, nitrate, carbonate and ammonium ions. [5]
Write the formulae of barium nitrate, aluminium sulfate, ammonium carbonate, magnesium hydroxide and aluminium oxide. [5]

Total: 10 marks

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SO₄^2-; OH^-; NO₃^-; CO₃^2-; NH₄⁺. One mark each.
Ba(NO₃)₂; Al₂(SO₄)₃; (NH₄)₂CO₃; Mg(OH)₂; Al₂O₃. One mark each.

Exam-style question 3

The ionic compounds X₂(SO₄)₃ and Y(OH)₂ contain simple ions X and Y.

Deduce the charges on X and Y. [2]
Write the formula of X nitrate. [1]
Write the formula of Y carbonate. [1]
Write the formula of ammonium sulfate. [1]
A student writes X₃SO₄. Explain why this is incorrect and give the correct sulfate formula. [2]

Total: 7 marks

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X³⁺ [1]; Y²⁺ [1].
X(NO₃)₃ [1].
YCO₃ [1].
(NH₄)₂SO₄ [1].
X₃SO₄ is not electrically neutral [1]. X₂(SO₄)₃ is correct [1].

✓ DONE WHENYou can balance charges, use brackets and simplify the ratio.

Giant ionic lattices and conductivity

Learning goalExplain conductivity from the mobility of ions.

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An ionic solid is a giant repeating arrangement of positive and negative ions. Strong electrostatic attractions act throughout the lattice, so a lot of energy is needed to separate the ions on melting.

For conductivity, ask whether charged particles can move. In the solid, the ions are held in fixed positions. When molten, the ions can move and carry charge.

Many ionic compounds dissolve in water, although not all do. If an ionic compound forms an aqueous solution, its mobile ions can carry charge. Do not explain ionic conductivity using moving electrons.

Giant ionic lattice: A regular repeating three-dimensional arrangement of oppositely charged ions.

State	Ions	Conducts?
Solid	Fixed in lattice positions	No: no mobile charge carriers
Molten	Mobile	Yes: ions carry charge
Aqueous, if dissolved	Hydrated and mobile	Yes: ions carry charge

🧪 TEST IT

Work on paper. Attempt each question before opening its answer. These are original CrackingChem questions, written in AQA style.

Exam-style question 4

Describe what is meant by a giant ionic lattice. [2]
Explain why magnesium oxide has a high melting point. [3]
Explain why solid magnesium oxide does not conduct electricity but molten magnesium oxide does. [3]

Total: 8 marks

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A regular, repeating three-dimensional arrangement [1] of oppositely charged ions [1].
A giant ionic lattice [1] with strong electrostatic attractions between Mg²⁺ and O^2- ions throughout [1]; much energy is needed to overcome these attractions [1].
Solid ions are fixed in lattice positions [1]. Molten ions are mobile [1] and carry charge [1].

✓ DONE WHENYou can explain solid and molten conductivity by naming the charged particles and their mobility.

Explaining ionic melting points

Learning goalLink ionic structure, attraction and energy to melting point.

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A high melting point needs an explanation of which particles attract and why separating them needs energy. For an ionic compound, those particles are oppositely charged ions.

Higher ion charges and smaller ionic radii generally strengthen the electrostatic attraction. More energy is then needed to overcome the attractions, so the melting point tends to be higher.

Use the actual ions in a comparison. Saying that one substance has 'stronger bonds' without identifying the attraction leaves out the chemistry.

Structure → particles → attraction → energy → property

Worked example

Why sodium chloride has a high melting point

Explain why sodium chloride has a high melting point. Refer to its structure, the particles present and the attractions between them.

Worked answer

NaCl has a giant ionic lattice.
Strong electrostatic attractions act between oppositely charged Na⁺ and Cl^- ions throughout the lattice.
Much energy is needed to overcome these attractions.

Therefore sodium chloride has a high melting point.

Worked example

Comparing ion charges: MgO and NaF

MgO and NaF have similar ion sizes and comparable lattice arrangements. Why does MgO have the higher melting point?

Worked answer

Both have giant ionic lattices. MgO contains Mg²⁺ and O^2-; NaF contains Na⁺ and F^-.
The ion sizes are similar, so focus on charge: the ions in MgO have larger charges.
The electrostatic attractions between opposite ions are stronger in MgO.
More energy is needed to overcome these attractions on melting.

Higher charges → stronger electrostatic attraction → more energy → higher melting point.

Worked example

Comparing ion sizes: NaCl and KCl

NaCl and KCl have the same ion charges and comparable lattice arrangements. Na⁺ is smaller than K⁺. Why does NaCl have the higher melting point?

Worked answer

Both contain a 1+ metal ion and the same Cl^- ion, so charge does not explain the difference.
The smaller Na⁺ ion allows the centres of opposite ions to be closer together.
The electrostatic attraction between the ions is stronger in NaCl.
More energy is needed to overcome these attractions on melting.

Smaller ions, with charge unchanged → stronger electrostatic attraction → higher melting point.

EXAM TRAP“Lots of strong bonds” does not name the ions or the electrostatic attraction.

🧪 TEST IT

Work on paper. Attempt each question before opening its answer. These are original CrackingChem questions, written in AQA style.

Quick check 5

Which has the higher melting point, CaO or KCl? Explain your answer. [3]

Total: 3 marks

Show answer & marking points

CaO has the higher melting point. Ca²⁺ and O^2- have larger charges than K⁺ and Cl^- [1], giving stronger electrostatic attractions between oppositely charged ions [1]. More energy is required to overcome these attractions when CaO melts [1].

Quick check 6

NaF and NaCl have comparable lattice arrangements. Explain why NaF has a higher melting point than NaCl. [3]

Total: 3 marks

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The smaller F^- ion allows the centres of opposite ions to be closer together [1]. With the same ion charges, electrostatic attractions are stronger in NaF [1], so more energy is needed to overcome them on melting [1].

Exam-style question 7

A solid oxide M₂O₃ melts at 2050 °C, does not conduct as a solid and conducts when molten.

Deduce the charge on M ions. [1]
Deduce the structure type. [1]
Explain all three physical-property observations. [5]
Write the formulae of M nitrate and M hydroxide. [2]

Total: 9 marks

Show answer & marking points

M³⁺ [1].
Giant ionic [1].
A giant lattice of M³⁺ and O^2- ions [1]; strong electrostatic attractions [1] require much energy to overcome [1]. Solid ions are fixed, so no conduction [1]; molten ions are mobile and carry charge [1].
M(NO₃)₃ [1]; M(OH)₃ [1].

✓ DONE WHENYou can explain high melting point using ions, electrostatic attraction and energy.

Ionic bonding & formulae: AQA A-level Chemistry