9.2 Periodicity of chemical properties of the elements in Period 3
- Syllabus
- 9701–2028–2029
- Topic
- 9.2
- Level
- AS
Learning objectives
9.2.1Reactions of the elements with oxygen (to• Describe/write equations: the reactions of the elements with oxygen (to give Na2O, MgO, Al2O3, P4O10, SO2), chlorine (to give NaCl, MgCl2, Al Cl 3, SiCl4, PCl5) and water (Na and Mg only)9.2.2The variation in the oxidation number• State/explain: the variation in the oxidation number of the oxides (Na2O, MgO, Al2O3, P4O10, SO2 and SO3 only) and chlorides (NaCl, MgCl2, Al Cl 3, SiCl4, PCl5 only) in terms of their outer shell (valence) electrons9.2.3Reactions/equations: the reactions, if any,• Describe/write equations: the reactions, if any, of the oxides Na2O, MgO, Al2O3, SiO2, P4O10, SO2 and SO3 with water, including the likely pHs of the solutions obtained9.2.4Reactions/equations: the acid / base behaviour• Describe/explain/write equations: 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)9.2.5Reactions of the chlorides NaCl, MgCl2, Al• Describe/explain/write equations: the reactions of the chlorides NaCl, MgCl2, Al Cl 3, SiCl4, PCl5 with water including the likely pHs of the solutions obtained9.2.6Variations and trends in 9.2.2, 9.2.3, 9.2.4• Explain the variations and trends in 9.2.2, 9.2.3, 9.2.4 and 9.2.5 in terms of bonding and electronegativity9.2.7Suggest the types of chemical bonding present• Suggest the types of chemical bonding present in the chlorides and oxides from observations of their chemical and physical properties
Period 3 elements form a defined set of oxides and chlorides
| Element | Reaction with oxygen | Main required product |
|---|---|---|
| Na | 4Na + O₂ → 2Na₂O | sodium oxide |
| Mg | 2Mg + O₂ → 2MgO | magnesium oxide |
| Al | 4Al + 3O₂ → 2Al₂O₃ | aluminium oxide |
| P | P₄ + 5O₂ → P₄O₁₀ | phosphorus(V) oxide |
| S | S + O₂ → SO₂ | sulfur dioxide |
| Element | Reaction with chlorine | Main required product |
|---|---|---|
| Na | 2Na + Cl₂ → 2NaCl | sodium chloride |
| Mg | Mg + Cl₂ → MgCl₂ | magnesium chloride |
| Al | 2Al + 3Cl₂ → 2AlCl₃ | aluminium chloride |
| Si | Si + 2Cl₂ → SiCl₄ | silicon(IV) chloride |
| P | P₄ + 10Cl₂ → 4PCl₅ | phosphorus(V) chloride |
| Element | Reaction with water | What is observed |
|---|---|---|
| Na | 2Na + 2H₂O → 2NaOH + H₂ | vigorous reaction; alkaline solution and hydrogen form |
| Mg, cold water | Mg + 2H₂O → Mg(OH)₂ + H₂ | very slow; a little hydrogen and sparingly soluble Mg(OH)₂ form |
| Mg, steam | Mg + H₂O(g) → MgO + H₂ | heated magnesium reacts much more readily |
Learn the products named in the syllabus rather than every possible oxide or chloride. The required phosphorus chloride is PCl₅, and only sodium and magnesium reactions with water are assessed in this outcome.
Oxidation numbers rise as more outer-shell electrons are used
Assign O an oxidation number of −2 and Cl an oxidation number of −1 in these compounds. The oxidation numbers in a neutral formula must sum to zero, so the Period 3 element has the balancing positive value.
| Compound series | Oxidation number of the Period 3 element |
|---|---|
| Na₂O, MgO, Al₂O₃ | Na +1, Mg +2, Al +3 |
| P₄O₁₀ | P +5 |
| SO₂, SO₃ | S +4, S +6 |
| NaCl, MgCl₂, AlCl₃, SiCl₄, PCl₅ | Na +1, Mg +2, Al +3, Si +4, P +5 |
Across Na to P, the number of outer-shell electrons increases from one to five. In the listed highest oxides and chlorides, progressively more of these electrons are transferred or shared with the more electronegative O or Cl, so the maximum positive oxidation number rises from +1 to +5. Sulfur then shows both +4 in SO₂ and +6 in SO₃ because four or all six of its outer-shell electrons are involved in the oxidation-number accounting.
Oxidation number is electron bookkeeping, not the actual charge on an atom in a covalent molecule. Do not force SO₂ and SO₃ into one value: sulfur is +4 in SO₂ but +6 in SO₃.
Water reveals the basic-to-acidic oxide transition
| Oxide | Reaction with water, if any | Likely pH of resulting mixture |
|---|---|---|
| Na₂O | Na₂O + H₂O → 2NaOH | strongly alkaline, about 13–14 |
| MgO | MgO + H₂O → Mg(OH)₂ | mildly alkaline, about 9–10 |
| Al₂O₃ | no reaction | about 7 |
| SiO₂ | no reaction | about 7 |
| P₄O₁₀ | P₄O₁₀ + 6H₂O → 4H₃PO₄ | acidic, about 2 |
| SO₂ | SO₂ + H₂O ⇌ H₂SO₃ | acidic, about 2–3 |
| SO₃ | SO₃ + H₂O → H₂SO₄ | strongly acidic, about 1 |
The ionic oxides on the left supply O²⁻, which accepts protons from water and produces OH⁻. The covalent non-metal oxides on the right react with water to form oxoacids. Al₂O₃ and SiO₂ do not react with water, so their acid-base character must be tested with other reagents.
No reaction with water does not mean an oxide has no acid-base behaviour: Al₂O₃ is amphoteric and SiO₂ is acidic when tested with suitable acid or base reagents.
Oxides and hydroxides change from basic through amphoteric to acidic
| Species | Behaviour | Diagnostic reaction |
|---|---|---|
| Na₂O, MgO | basic oxides | Na₂O + 2HCl → 2NaCl + H₂O; MgO + 2HCl → MgCl₂ + H₂O |
| Al₂O₃ | amphoteric oxide | Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O; Al₂O₃ + 2NaOH + 3H₂O → 2NaAl(OH)₄ |
| SiO₂ | acidic oxide | SiO₂ + 2NaOH → Na₂SiO₃ + H₂O |
| P₄O₁₀ | acidic oxide | P₄O₁₀ + 12NaOH → 4Na₃PO₄ + 6H₂O |
| SO₂ | acidic oxide | SO₂ + 2NaOH → Na₂SO₃ + H₂O |
| SO₃ | acidic oxide | SO₃ + 2NaOH → Na₂SO₄ + H₂O |
| Hydroxide | Behaviour | Equation evidence |
|---|---|---|
| NaOH | soluble strong base | NaOH + HCl → NaCl + H₂O |
| Mg(OH)₂ | sparingly soluble base | Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O |
| Al(OH)₃ | amphoteric | Al(OH)₃ + 3HCl → AlCl₃ + 3H₂O; Al(OH)₃ + NaOH → NaAl(OH)₄ |
A basic oxide or hydroxide reacts with acid; an acidic oxide reacts with sodium hydroxide; an amphoteric oxide or hydroxide does both. Across the period, the dominant behaviour therefore changes basic → amphoteric → acidic.
For the base reactions in this syllabus outcome, use sodium hydroxide as the base reagent. Amphoteric means reaction with both acids and bases; it does not mean neutral or unreactive.
Period 3 chlorides range from dissolution to hydrolysis
| Chloride added to water | Main change and equation | Likely pH |
|---|---|---|
| NaCl | dissolves: NaCl(s) → Na⁺(aq) + Cl⁻(aq) | about 7 |
| MgCl₂ | dissolves: MgCl₂(s) → Mg²⁺(aq) + 2Cl⁻(aq); hydrated Mg²⁺ makes the solution slightly acidic | about 6–7 |
| AlCl₃ | forms hydrated Al³⁺, which hydrolyses: [Al(H₂O)₆]³⁺ + H₂O ⇌ [Al(H₂O)₅(OH)]²⁺ + H₃O⁺ | about 3 |
| SiCl₄ | vigorous hydrolysis: SiCl₄ + 2H₂O → SiO₂ + 4HCl | about 2 |
| PCl₅ | complete hydrolysis: PCl₅ + 4H₂O → H₃PO₄ + 5HCl | about 2 |
NaCl and MgCl₂ simply form colourless solutions. AlCl₃ gives an acidic solution. SiCl₄ hydrolysis produces acidic hydrogen chloride and solid hydrated silica/SiO₂, often seen with steamy acidic fumes; PCl₅ also hydrolyses vigorously to acidic products.
Dissolving separates pre-existing ions; hydrolysis changes a species by reaction with water. Do not substitute PCl₃ for PCl₅: this outcome names PCl₅.
Electronegativity and bonding explain the chemical trends
Electronegativity increases across Period 3. The electronegativity difference between the Period 3 element and O or Cl therefore decreases, so bonding changes from predominantly ionic on the left to increasingly covalent toward the right. The small, highly charged Al³⁺ ion strongly polarises nearby electron clouds, giving aluminium compounds appreciable covalent character at the transition.
| Observed trend | Bonding explanation | Chemical consequence |
|---|---|---|
| Na₂O and MgO are basic | ionic lattices contain O²⁻ | O²⁻ accepts H⁺ and produces OH⁻ in water |
| Al₂O₃ and Al(OH)₃ are amphoteric | aluminium lies at the ionic–covalent transition | both acid and strong base can react |
| Si, P and S oxides are acidic | covalent central-atom–oxygen bonding; the central atom withdraws electron density | water forms oxoacids where reaction occurs, and the oxides react with NaOH |
| chloride solutions become more acidic | rising cation charge density, then covalent E–Cl bonds, increasingly polarise or react with water | hydrolysis generates H₃O⁺ or HCl |
For oxides of the same element, a higher positive oxidation number makes the central atom more electron-withdrawing. This is why SO₃, with S at +6, gives a more strongly acidic oxide than SO₂, with S at +4.
Do not explain every trend with electronegativity alone. Use electronegativity to infer ionic versus covalent character, then connect that bonding to the relevant particles, polarisation and reaction with water, acid or base.
Use a cluster of observations to infer bonding and structure
| Observation cluster | Best inference | Period 3 examples |
|---|---|---|
| high melting point; brittle solid; conducts when molten or in aqueous solution but not as a solid | giant ionic lattice with mobile ions only when free to move | NaCl, MgCl₂; Na₂O and MgO |
| very high melting point; hard; insoluble; no electrical conduction; no reaction with water | giant covalent network | SiO₂ |
| low melting/boiling point or volatility; no electrical conduction; hydrolysis with water | simple molecular covalent substance | SiCl₄ and molecular P/S oxides; covalent chlorides hydrolyse rather than merely dissociate |
| high-melting oxide that reacts with both acid and base | extended lattice at the ionic–covalent boundary | Al₂O₃ |
Infer in three steps: identify whether charged particles can move, decide whether melting separates ions/atoms or intact molecules, then use dissolution or hydrolysis as supporting chemical evidence. A conclusion is strongest when several observations point to the same model.
AlCl₃ has covalent character and often exists as Al₂Cl₆ molecules; PCl₅ has phase-dependent structure, so do not infer every chloride from one physical observation alone. State the model justified by the given data and use hydrolysis as additional evidence for covalent character.