Flashcards · 9701 · AS Topic 9

The Periodic Table: chemical periodicity (AS Level) flashcards

15 cards on AS Topic 9, subtopics 9.1 to 9.3. Pick a subtopic or study them all.

Study mode needs JavaScript. Every card is listed below, so you can still cover the answers and test yourself.

All 15 cards

For a quick read-through or printing. Cover the right-hand side to test yourself.

Trend in atomic radius across Period 3 and the reason: Decreases: nuclear charge increases while electrons are added to the same shell with roughly constant shielding, so outer electrons are pulled closer.
Trend in ionic radius across Period 3: Decreases from Na⁺ to Al³⁺, jumps up sharply to P³⁻, then decreases to Cl⁻. The anions have one more occupied shell than the cations; within each set, nuclear charge rises for the same electron arrangement.
Explain the melting point pattern across Period 3: Na → Al: metallic, rising (more delocalised electrons per ion, smaller ions). Si: giant covalent, highest (many strong covalent bonds). P₄, S₈, Cl₂, Ar: simple molecular/atomic, low (weak id-id forces only; S₈ > P₄ as it is larger).
Why does melting point fall sharply after silicon?: The structure changes from giant covalent to simple molecular, so only weak intermolecular forces must be overcome. It is not because the bonds get weaker.
Trends in first ionisation energy and electronegativity across Period 3: Both increase (greater nuclear charge, smaller radius, similar shielding). First IE dips at Al and at S.
Electrical conductivity across Period 3: Na, Mg and Al are metallic and conduct (increasingly, as more delocalised electrons per atom); Si is a semiconductor; P, S, Cl and Ar do not conduct.

Period 3 oxides with water: products and pH: Na₂O → NaOH, pH 13–14. MgO → slightly soluble Mg(OH)₂, pH 9–10. Al₂O₃ and SiO₂: insoluble, pH 7. P₄O₁₀ → H₃PO₄, pH 1–2. SO₂/SO₃ → H₂SO₃/H₂SO₄, pH 1–3.
Show that Al₂O₃ is amphoteric: It reacts with acids and with bases: Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O Al₂O₃ + 2NaOH → 2NaAlO₂ + H₂O
Period 3 chlorides with water: NaCl: dissolves, no reaction, pH 7. MgCl₂: dissolves, slightly acidic, pH about 6. AlCl₃: hydrolyses, pH about 3. SiCl₄ and PCl₅: violent hydrolysis, misty HCl fumes, pH about 2.
Why does the behaviour of the chlorides change from dissolving to hydrolysis across Period 3?: The electronegativity difference from chlorine shrinks, so bonding changes from ionic (dissolves as ions) to covalent (water attacks the Si–Cl or P–Cl bond).
Oxidation numbers of Period 3 elements in their oxides: Na₂O +1, MgO +2, Al₂O₃ +3, SiO₂ +4, P₄O₁₀ +5, SO₃ +6. They rise as more outer electrons are used in bonding.
Why is MgO in water only about pH 9–10?: Mg(OH)₂ is only sparingly soluble, so few hydroxide ions enter solution.

Element X forms an oxide giving a strongly acidic solution and a chloride that fumes in moist air. Deduce X.: A violently hydrolysing chloride is covalent (like SiCl₄ or PCl₅). SiO₂ does not react with water, so a strongly acidic oxide points to P₄O₁₀: X is phosphorus.
How can you use periodic patterns to predict an unfamiliar element?: Use its position: metals on the left give ionic, basic oxides and chlorides that dissolve; non-metals on the right give covalent, acidic oxides and chlorides that hydrolyse. Trends in radius, IE and electronegativity follow from nuclear charge, shells and shielding.
An unknown element has a high-melting oxide that is basic and a chloride that dissolves to give pH 7. Where is it in the period?: On the left (Group 1 type): an ionic, basic oxide and an ionic chloride that dissolves without hydrolysis, like Na₂O and NaCl.