Why does MgO have a higher melting point than Na₂O?

Mg²⁺ has a greater charge and smaller radius, so the attraction is stronger.

Why does SiO₂ have a very high melting point?

It is giant covalent, so many strong covalent bonds must be broken.

Why do P₄O₁₀ and SO₃ have low melting points?

They are simple molecular, so only weak intermolecular forces are overcome.

What is the pH of the solution from Al₂O₃ and water?

About 7, because Al₂O₃ is insoluble.

Write the equation for P₄O₁₀ with water.

P₄O₁₀ + 6H₂O → 4H₃PO₄

Write the equations for SO₂ and SO₃ with water.

SO₂ + H₂O → H₂SO₃; SO₃ + H₂O → H₂SO₄

Describe the structure of H₂SO₄.

Sulfur bonded to four oxygen atoms: two S=O and two S–OH.

What does amphoteric mean?

Reacts with both acids and bases.

Write the equation for Al₂O₃ with hot concentrated NaOH solution.

Al₂O₃ + 2NaOH + 3H₂O → 2NaAl(OH)₄

Why do oxides change from basic to acidic across the period?

Bonding changes from ionic (O²⁻ accepts protons) to covalent (forms acids with water).

Period 3 oxides: structure and acid-base characterAQA A-Level Chemistry: Flashcards

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What structure and bonding does MgO have?

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What structure and bonding does MgO have?: Giant ionic lattice with strong electrostatic attraction between Mg²⁺ and O²⁻ ions.
Why does MgO have a higher melting point than Na₂O?: Mg²⁺ has a greater charge and smaller radius, so the attraction is stronger.
Why does SiO₂ have a very high melting point?: It is giant covalent, so many strong covalent bonds must be broken.
Why do P₄O₁₀ and SO₃ have low melting points?: They are simple molecular, so only weak intermolecular forces are overcome.
Write the equation for Na₂O with water.: Na₂O + H₂O → 2NaOH
What is the pH of the solution from Al₂O₃ and water?: About 7, because Al₂O₃ is insoluble.
Write the equation for P₄O₁₀ with water.: P₄O₁₀ + 6H₂O → 4H₃PO₄
Write the equations for SO₂ and SO₃ with water.: SO₂ + H₂O → H₂SO₃; SO₃ + H₂O → H₂SO₄
Describe the structure of H₂SO₄.: Sulfur bonded to four oxygen atoms: two S=O and two S–OH.
What is the anion formed from H₃PO₄ fully neutralised?: PO₄³⁻, phosphate(V).
What does amphoteric mean?: Reacts with both acids and bases.
Write the equation for Al₂O₃ with hot concentrated NaOH solution.: Al₂O₃ + 2NaOH + 3H₂O → 2NaAl(OH)₄
Write the equation for MgO with hydrochloric acid.: MgO + 2HCl → MgCl₂ + H₂O
Why do oxides change from basic to acidic across the period?: Bonding changes from ionic (O²⁻ accepts protons) to covalent (forms acids with water).

Exam questions on Period 3 oxides: structure and acid-base character

A student is given samples of four Period 3 oxides, sodium oxide, magnesium oxide, silicon dioxide and phosphorus(V) oxide, and is asked to compare their melting points.
Explain why the melting point of magnesium oxide is higher than that of sodium oxide.2 marks
A student adds small amounts of sodium oxide, aluminium oxide and phosphorus(V) oxide to separate beakers of water and tests the pH of each mixture with universal indicator.
Write an equation for the reaction of sodium oxide with water and explain why the resulting solution is alkaline.2 marks
A technician is preparing a safety sheet for the oxides of Period 3. She needs equations for the acids formed when phosphorus(V) oxide, sulfur dioxide and sulfur trioxide dissolve in water, and a description of the structures of some of the acids and anions involved.
Write equations for the reactions of phosphorus(V) oxide, sulfur dioxide and sulfur trioxide with water.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).