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Periodicity and Period 3
Unit C12 · 3 lessons.
Across Period 3 the atomic radius falls, the first ionisation energy rises with two dips, and the melting point climbs to silicon and then drops. The elements react with oxygen and with water in ways that follow from their bonding, and the oxides run from basic through amphoteric to acidic.
The slip is in the melting points. Silicon's is highest because its giant covalent (macromolecular) structure must have strong covalent bonds broken, not because it has metallic bonding. For phosphorus, sulfur and chlorine the order follows the size of the molecules, P₄, S₈ and Cl₂, and so the strength of the induced dipole–dipole forces (van der Waals forces) between them.
On the written papers it is set on AQA Papers 1 and 3, OCR A Papers 1 and 3, Edexcel Papers 1 and 3 and Cambridge Papers 1 and 2.
Specification sections: AQA 3.2.1.1, 3.2.1.2, 3.2.4 · OCR A 3.1.1 · Edexcel 1.13 to 1.26 · CAIE 9.1, 9.2
Notes
Illustrated notes for each topic in this unit, with worked examples and a checklist of objectives.
- Periodicity: trends across Period 3 and the blocks of the table
Periodicity as a repeating pattern, the s, p, d and f blocks and classifying an element from its electron configuration, the trends across Period 3 in atomic radius, first ionisation energy and melting point with the structure and bonding behind each, electrical conductivity and ionic radius, and the same trends in Period 2 in outline.
- The Period 3 elements: reactions with oxygen and water
The reactions of sodium and magnesium with water and of magnesium with steam, with equations, observations and the pH of the resulting solution; the reactions of sodium, magnesium, aluminium, silicon, phosphorus and sulfur with oxygen and what is seen; making sulfur trioxide; the oxidation states and formulae of Na₂O, MgO, Al₂O₃, SiO₂, P₄O₁₀, SO₂ and SO₃; and, for Cambridge, the reactions of sodium to phosphorus with chlorine and the formulae, oxidation states and bonding of NaCl, MgCl₂, AlCl₃, SiCl₄ and PCl₅.
- The Period 3 oxides: structure, bonding and acid–base character
The melting points of Na₂O, MgO, Al₂O₃, SiO₂, P₄O₁₀, SO₂ and SO₃ explained by structure and bonding; the reactions of the oxides with water and the pH of each solution; the structures of the acids and anions formed from P₄O₁₀, SO₂ and SO₃; basic, amphoteric and acidic oxides; equations for each oxide with acids and bases, including aluminium oxide and aluminium hydroxide both ways; and, for Cambridge, the reactions of NaCl, MgCl₂, AlCl₃, SiCl₄ and PCl₅ with water and the pH of each solution.
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