The Periodic Table: chemical periodicity

• Left to right: metallic → covalent network → molecules; ionic oxide → covalent oxide; alkali → acid. The oxidation number counts the outer electrons used.
• Metallic, giant covalent, molecular, monatomic. Ionic oxide alkaline, covalent oxide acidic, Al₂O₃ both. Ionic chloride dissolves, covalent chloride fumes. Oxidation number = outer electrons used.

• 9.1.1 I can describe — describe qualitatively (and indicate the periodicity in) the variations in atomic radius, ionic radius, melting point and electrical conductivity of the elements
• 9.1.2 I can explain — explain the variation in melting point and electrical conductivity in terms of the structure and bonding of the elements
• 9.2.1 I can describe — describe, and write equations for, the reactions of the elements with oxygen (to give Na₂O, MgO, Al₂O₃, P₄O₁₀, SO₂), chlorine (to give NaCl, MgCl₂, AlCl₃, SiCl₄, PCl₅) and water (Na and Mg only)
• 9.2.2 I can state — state and explain the variation in the oxidation number of the oxides (Na₂O, MgO, Al₂O₃, P₄O₁₀, SO₂ and SO₃ only) and chlorides (NaCl, MgCl₂, AlCl₃, SiCl₄, PCl₅ only) in terms of their outer shell (valence) electrons
• 9.2.3 I can describe — describe, and write equations for, the reactions, if any, of the oxides Na₂O, MgO, Al₂O₃, SiO₂, P₄O₁₀, SO₂ and SO₃ with water, including the likely pHs of the solutions obtained
• 9.2.4 I can describe — describe, explain, and write equations for, the acid/base behaviour of the oxides Na₂O, MgO, Al₂O₃, P₄O₁₀, SO₂ and SO₃ and the hydroxides NaOH, Mg(OH)₂ and Al(OH)₃ including, where relevant, amphoteric behaviour in reactions with acids and bases (sodium hydroxide only)
• 9.2.5 I can describe — describe, explain, and write equations for, the reactions of the chlorides NaCl, MgCl₂, AlCl₃, SiCl₄, PCl₅ with water including the likely pHs of the solutions obtained
• 9.2.6 I can explain — explain the variations and trends in 9.2.2, 9.2.3, 9.2.4 and 9.2.5 in terms of bonding and electronegativity
• 9.2.7 I can suggest — suggest the types of chemical bonding present in the chlorides and oxides from observations of their chemical and physical properties
• 9.3.1 I can predict — predict the characteristic properties of an element in a given group by using knowledge of chemical periodicity
• 9.3.2 I can deduce — deduce the nature, possible position in the Periodic Table and identity of unknown elements from given information about physical and chemical properties

The same chemistry is a qualitative-analysis test. The syllabus's notes for Al³⁺(aq) with NaOH(aq) read “white ppt., soluble in excess”: the first drops precipitate Al(OH)₃, and further NaOH dissolves it because the hydroxide is amphoteric. Mg²⁺(aq) gives “white ppt., insoluble in excess”, because Mg(OH)₂ is basic only. Aluminium's chemistry in aqueous solution beyond this is Topic 28.

• “Sulfur is in Group 16, so it is +6 in SO₂.” Correct The oxidation number is the number of outer electrons used in bonding, so SO₂ is +4 and SO₃ is +6. In SO₂ sulfur uses four of its six outer electrons and keeps one lone pair.
• “Ionic radius falls smoothly from Na⁺ to Cl⁻.” Correct It falls Na⁺ > Mg²⁺ > Al³⁺, jumps up to P³⁻, then falls P³⁻ > S²⁻ > Cl⁻. The anions have one more occupied shell than the cations.
• “Every metal in Period 3 melts higher than every non-metal.” Correct Sodium (about 371 K) melts below sulfur (about 392 K). Sodium's metallic bonding, one delocalised electron per Na⁺, is weak; S₈ molecules have 128 electrons each.
• “Silicon melts high because of strong intermolecular forces.” Correct Silicon has no molecules. It is a giant covalent lattice, and melting it breaks covalent bonds.
• “Burning sulfur gives SO₃.” Correct Burning sulfur gives SO₂. SO₃ is made only by the catalysed oxidation of SO₂ in the Contact process (Topic 7).
• “Al₂O₃ dissolves in water to give a neutral solution.” Correct Al₂O₃ and SiO₂ do not dissolve at all; the water stays at pH 7 with the solid at the bottom.
• “NaCl reacts with water and gives HCl.” Correct Ionic chlorides dissolve without reaction (pH 7; MgCl₂ 6.5). Only the covalent chlorides, AlCl₃, SiCl₄ and PCl₅, are hydrolysed and release HCl.
• “Atomic radius decreases across Period 3 because there are more electrons.” Repair The extra electrons go into the same shell and shield each other poorly. The cause is the rising nuclear charge with almost constant shielding by the inner electrons.
• “Ionic radius decreases smoothly from Na⁺ to Cl⁻.” Repair It falls Na⁺ > Mg²⁺ > Al³⁺, jumps up at P³⁻ because the anions have an extra occupied shell, then falls P³⁻ > S²⁻ > Cl⁻.
• “Silicon has a high melting point because of strong intermolecular forces.” Repair Silicon has no molecules. It is a giant covalent lattice, and covalent bonds are broken on melting.
• “Sulfur melts above phosphorus because S–S bonds are stronger than P–P bonds.” Repair No covalent bond breaks when a molecular solid melts. S₈ has more electrons (128) than P₄ (60), so its instantaneous dipole–induced dipole forces are stronger.
• “Aluminium conducts best because it is the most reactive metal.” Repair Reactivity is irrelevant. Aluminium has three delocalised electrons per atom, the most of the three metals, so it has the most charge carriers.
• S(s) + O₂(g) → SO₃(g), for burning sulfur. Repair Burning gives SO₂: S(s) + O₂(g) → SO₂(g). SO₃ needs the catalysed oxidation of SO₂ in the Contact process (Topic 7). The equation as written does not even balance.
• Mg(s) + H₂O(g) → Mg(OH)₂(s) + H₂(g), for magnesium with steam. Repair With steam the product is the oxide: Mg(s) + H₂O(g) → MgO(s) + H₂(g). The hydroxide forms, very slowly, only with cold liquid water. (The equation as written does not balance either.)
• “Sulfur in SO₂ is +6 because it is in Group 16.” Repair The oxidation number is the number of outer electrons used: four in SO₂ (+4), six in SO₃ (+6).
• “Al₂O₃ dissolves in water to give a neutral solution.” Repair It does not dissolve at all; the water stays at pH 7.
• “MgO gives pH 14, like Na₂O.” Repair Mg(OH)₂ is only sparingly soluble, so the solution is only weakly alkaline: pH about 9–10.
• “Amphoteric means it reacts with water in two ways.” Repair It means the oxide or hydroxide reacts with both acids and bases. Al₂O₃ does not react with water at all.
• “NaCl hydrolyses in water to give HCl.” Repair Ionic chlorides dissolve without reaction. Only the covalent chlorides, AlCl₃, SiCl₄ and PCl₅, are hydrolysed.
• SiCl₄ + H₂O → Si(OH)₄ + HCl. Repair Unbalanced, and not the product this syllabus uses: SiCl₄(l) + 2H₂O(l) → SiO₂(s) + 4HCl(aq).
• “Covalent oxides are acidic because they contain hydrogen.” Repair They contain no hydrogen and no O²⁻ ion. With water their non-metal atom forms an oxoacid, and it is that acid which releases H⁺.
• “It is a solid, so it must be ionic.” Repair AlCl₃ and PCl₅ are solids and covalent. Deduce the bonding from two agreeing observations, conduction when molten and behaviour in water, each with its evidence.

• Read the command word before you decide how much to write. This syllabus has twenty-two of them: analyse, calculate, compare, consider, contrast, deduce, define, demonstrate, describe, determine, discuss, evaluate, examine, explain, give, identify, justify, predict, show (that), sketch, state and suggest. Comment, estimate, name and outline are not among them: where a question wants something named it says identify, which the syllabus glosses as “name/select/recognise”. State and give want a fact and nothing more. Describe wants the points or the features. Explain wants the reasons and the relationships — a describe-level answer to an explain question is incomplete however well written it is. Deduce and determine want a conclusion reached from the information given, with the reasoning visible.
• Interleave with the chapters that use this one. When you reach Topic 10 (Group 2), predict the oxides and hydroxides from magnesium first, then check. At Topic 11 (Group 17), redo the bromine prediction of section J. At Topic 28 (transition elements), re-read why this chapter stopped at “white ppt., soluble in excess” for Al³⁺. Recalling a topic inside a new context is worth more than another pass over this chapter on its own; at A Level, Paper 4 assumes the whole of the AS content, so nothing here is ever finished with.

• Cambridge International AS & A Level Chemistry 9701 has five components. Topic 9 is AS Level content, so it is examined in Papers 1, 2 and 3. AS Level content: examined in Paper 1 (multiple choice), Paper 2 (AS structured) and, as practical context, Paper 3. Assumed knowledge for Papers 4 and 5. AS Level candidates take Papers 1, 2 and 3; A Level candidates take all five, either staged over two years (Papers 1–3 in year one, Papers 4 and 5 in year two) or together in one series. Examinations are available in the June and November series, and in March in India.
• Across both the AS Level and the A Level the assessment objectives are weighted AO1 40% (knowledge and understanding), AO2 40% (handling, applying and evaluating information) and AO3 20% (experimental skills and investigations). AS candidates are graded a–e; A Level candidates A*–E.
• A multiple-choice item on this topic can turn on one comparison: which ion is largest, which element melts highest, which oxide is amphoteric, which chloride gives the lowest pH, which oxidation number is right. A structured question asks you to describe a trend across Period 3, explain it from structure and bonding, write balanced equations with state symbols, state the likely pH of a solution, or deduce the bonding or the identity of an unknown from data.
• The Pauling electronegativity values are in the Data section and are given to you; you compute the differences. Melting points, radii and conductivities are not in the Data section: a question that needs a number supplies it. Mole calculations appear through the equations of this topic, such as the mass of NaOH that neutralises the acid from a given mass of P₄O₁₀.
• Paper 3's observational problem can give an unknown oxide or chloride to test with water, universal indicator, dilute acid and aqueous NaOH. The quantitative procedure this topic supplies is gravimetric: heating a solid in a crucible to constant mass, as in finding the formula of magnesium oxide. A Paper 5-style question could ask you to plan it, process the masses and evaluate the errors.
• Read the command word before you decide how much to write. This syllabus has twenty-two of them: analyse, calculate, compare, consider, contrast, deduce, define, demonstrate, describe, determine, discuss, evaluate, examine, explain, give, identify, justify, predict, show (that), sketch, state and suggest. Comment, estimate, name and outline are not among them: where a question wants something named it says identify, which the syllabus glosses as “name/select/recognise”. State and give want a fact and nothing more. Describe wants the points or the features. Explain wants the reasons and the relationships — a describe-level answer to an explain question is incomplete however well written it is. Deduce and determine want a conclusion reached from the information given, with the reasoning visible.