A-Level Chemistry: Period 3 Elements and Oxides

Period 3 is where A-level inorganic chemistry shows most clearly how bonding controls properties. Across the period the oxides change from giant ionic solids to giant covalent to small covalent molecules, and their behaviour changes with them, from strongly basic to amphoteric to strongly acidic. This Year 13 material trains you to describe those changes and, more importantly, to explain them.

It starts with the elements. Sodium reacts vigorously with cold water to give a strongly alkaline solution, while magnesium reacts only very slowly with cold water and needs steam for a fast reaction that gives magnesium oxide and hydrogen. When heated in oxygen, sodium burns with a yellow-orange flame, magnesium with a brilliant white flame, phosphorus with a white flame and clouds of white phosphorus(V) oxide, and sulfur with a blue flame to form sulfur dioxide. You write balanced equations for the formation of Na₂O, MgO, Al₂O₃, SiO₂, P₄O₁₀ and SO₂.

The second part is about the oxides themselves. You explain the melting points in terms of structure and bonding: strong ionic lattices in sodium, magnesium and aluminium oxides (with MgO higher than Na₂O because of the smaller, more highly charged Mg²⁺ ion, and Al₂O₃ lower than a purely ionic model predicts because of covalent character), a giant covalent network in silicon dioxide, and weak forces between the small molecules of P₄O₁₀, SO₃ and SO₂. You then describe how each oxide behaves with water, with the approximate pH of the solution formed: sodium hydroxide at about pH 14, weakly alkaline magnesium hydroxide, no reaction for aluminium oxide and silicon dioxide, and strongly acidic solutions from the oxides of phosphorus and sulfur. You link this pattern to bonding (ionic oxides basic, covalent oxides acidic), show that aluminium oxide is amphoteric by writing its reactions with hydrochloric acid and with sodium hydroxide, and write neutralisation equations for the other oxides with given acids and bases, including silicon dioxide with hot concentrated alkali. The acids and anions formed, such as H₃PO₄ and PO₄³⁻, H₂SO₃ and SO₃²⁻, H₂SO₄ and SO₄²⁻, complete the topic.

Four formats are offered. The 12-question quiz mixes observations, equations and explanations. The flashcards give each element with oxygen and water and each oxide with water, with the pH. The written work has eight questions to answer by hand, with full equations and explanations, each with a reference answer. The oral exam lets you explain the trends aloud to an examiner, one question at a time.

The content is based on the A-level chemistry content on the properties of Period 3 elements and their oxides, as set out for example in the AQA A-level specification, which limits it to sodium to sulfur and these seven oxides. It is revision practice written by Zestly, not an exam board resource.

• Describe and write equations for the reactions of sodium and magnesium with water
• Describe the reactions of Na, Mg, Al, Si, P and S with oxygen and write equations for the oxides formed
• Explain the trend in melting points of the Period 3 oxides in terms of structure and bonding
• Describe the reactions of the oxides with water and give the approximate pH of each solution
• Explain the link between bonding and acid–base character, including amphoteric Al₂O₃
• Write equations for the oxides reacting with given acids and bases, and give the formulas of the acids and anions formed

Practice material written by Zestly, based on the A-level chemistry content on the properties of Period 3 elements and their oxides (for example AQA A-level Chemistry 7405, section 3.2.4).