Big picture: what changes across Period 3?

· Across Na → Mg → Al → Si → P → S, elements change from metals → metalloids/non-metals.
· Electronegativity increases across Period 3, so bonding in compounds changes from mostly ionic to mostly covalent.
· Oxides change from basic → amphoteric → acidic.
· Chlorides change from mostly ionic chlorides to covalent chlorides.
· Exam focus: write balanced equations, predict pH, explain using bonding + electronegativity, and link observations to ionic/covalent character.

Reactions of Period 3 elements with oxygen

· Sodium burns in oxygen to form sodium oxide: 4Na + O₂ → 2Na₂O.
· Magnesium burns with a bright white flame: 2Mg + O₂ → 2MgO.
· Aluminium forms aluminium oxide: 4Al + 3O₂ → 2Al₂O₃.
· Phosphorus forms phosphorus(V) oxide: P₄ + 5O₂ → P₄O₁₀.
· Sulfur burns with a blue flame to form sulfur dioxide: S + O₂ → SO₂.
· Oxides required: Na₂O, MgO, Al₂O₃, P₄O₁₀, SO₂; also know SO₃ for oxide acidity and oxidation number trends.

Reactions of Period 3 elements with chlorine

· Sodium chloride: 2Na + Cl₂ → 2NaCl.
· Magnesium chloride: Mg + Cl₂ → MgCl₂.
· Aluminium chloride: 2Al + 3Cl₂ → 2AlCl₃.
· Silicon tetrachloride: Si + 2Cl₂ → SiCl₄.
· Phosphorus(V) chloride: P₄ + 10Cl₂ → 4PCl₅.
· Trend: chlorides become less ionic and more covalent across the period as electronegativity of the Period 3 element increases.

Reactions of sodium and magnesium with water

· Sodium reacts vigorously with cold water to form an alkaline solution: 2Na + 2H₂O → 2NaOH + H₂.
· Observations for sodium: floats, melts into a ball, fizzes, moves rapidly, solution becomes strongly alkaline.
· Magnesium reacts very slowly with cold water: Mg + 2H₂O → Mg(OH)₂ + H₂.
· Magnesium reacts more readily with steam: Mg + H₂O(g) → MgO + H₂.
· Magnesium reaction is slower because Mg has a protective oxide layer and is less reactive than sodium.

Oxidation numbers in Period 3 oxides and chlorides

· Oxides: Na₂O = Na +1, MgO = Mg +2, Al₂O₃ = Al +3, P₄O₁₀ = P +5, SO₂ = S +4, SO₃ = S +6.
· Chlorides: NaCl = Na +1, MgCl₂ = Mg +2, AlCl₃ = Al +3, SiCl₄ = Si +4, PCl₅ = P +5.
· Oxidation number generally matches the number of outer shell electrons used in bonding.
· Across Period 3, the maximum oxidation number increases because elements have more valence electrons available for bonding.
· Key exam phrase: oxidation number increases across the period due to increasing number of outer shell electrons involved in bonding.

Reactions of Period 3 oxides with water

· Na₂O reacts with water to form strong alkali: Na₂O + H₂O → 2NaOH, pH about 14.
· MgO reacts slightly with water: MgO + H₂O → Mg(OH)₂, pH about 9–10.
· Al₂O₃ does not react with water.
· SiO₂ does not react with water.
· P₄O₁₀ reacts vigorously with water: P₄O₁₀ + 6H₂O → 4H₃PO₄, acidic solution.
· SO₂ dissolves/reacts with water: SO₂ + H₂O → H₂SO₃, acidic solution.
· SO₃ reacts with water: SO₃ + H₂O → H₂SO₄, strongly acidic solution.
· Trend: solutions change from alkaline → no reaction → acidic across Period 3.

This table summarises which oxides react with water, acid and base. It is especially useful for remembering basic, acidic and amphoteric behaviour in exam equations. Source

Acid/base behaviour of Period 3 oxides

· Basic oxides: Na₂O and MgO.
· Basic oxides react with acids to form salt + water.
· Na₂O + 2HCl → 2NaCl + H₂O.
· MgO + 2HCl → MgCl₂ + H₂O.
· Amphoteric oxide: Al₂O₃; it reacts with both acids and bases.
· With acid: Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O.
· With hot concentrated alkali: Al₂O₃ + 2NaOH + 3H₂O → 2NaAl(OH)₄.
· Acidic oxides: P₄O₁₀, SO₂, SO₃; they react with alkalis to form salt + water.
· P₄O₁₀ + 12NaOH → 4Na₃PO₄ + 6H₂O.
· SO₂ + 2NaOH → Na₂SO₃ + H₂O.
· SO₃ + 2NaOH → Na₂SO₄ + H₂O.
· SiO₂ is acidic but does not react with water; it reacts with hot concentrated alkali: SiO₂ + 2NaOH → Na₂SiO₃ + H₂O.

Hydroxides: NaOH, Mg(OH)₂ and Al(OH)₃

· NaOH is a strong base; it fully dissociates in water to give OH⁻ ions.
· Mg(OH)₂ is a weak/less soluble base; used to neutralise acids.
· Al(OH)₃ is amphoteric.
· With acid: Al(OH)₃ + 3HCl → AlCl₃ + 3H₂O.
· With aqueous sodium hydroxide: Al(OH)₃ + NaOH → NaAl(OH)₄.
· Exam point: amphoteric = reacts with both acids and bases.

Reactions of Period 3 chlorides with water

· NaCl: dissolves in water; no hydrolysis, solution approximately neutral.
· MgCl₂: dissolves in water; slight acidity may occur due to hydrated Mg²⁺, solution around pH 6–7.
· AlCl₃: hydrolyses in water; acidic solution, often around pH 3.
· Hydrated aluminium ion loses H⁺: [Al(H₂O)₆]³⁺ ⇌ [Al(H₂O)₅OH]²⁺ + H⁺.
· SiCl₄ hydrolyses violently with water, giving white fumes of HCl: SiCl₄ + 2H₂O → SiO₂ + 4HCl.
· PCl₅ hydrolyses with water: PCl₅ + 4H₂O → H₃PO₄ + 5HCl.
· Solutions from SiCl₄ and PCl₅ are strongly acidic due to HCl and acidic products.
· Trend: chlorides become more likely to hydrolyse across Period 3 as bonding becomes more covalent.

Explaining the trends using bonding and electronegativity

· Across Period 3, electronegativity increases, so compounds become more covalent.
· Metal oxides on the left contain O²⁻ ions, so they are basic because O²⁻ accepts H⁺.
· Non-metal oxides on the right are covalent acidic oxides; they react with water to form acids or react with bases to form salts.
· Al₂O₃ and Al(OH)₃ are amphoteric because aluminium bonding has both ionic and covalent character.
· NaCl and MgCl₂ are mainly ionic, so they mostly dissolve rather than hydrolyse.
· SiCl₄ and PCl₅ are covalent chlorides, so they hydrolyse with water to produce HCl, giving acidic fumes/solutions.
· Key exam explanation: increasing electronegativity difference? Metal chlorides/oxides are more ionic; smaller electronegativity difference with non-metals gives more covalent compounds.

Deducing bonding from chemical and physical properties

· Ionic oxides/chlorides usually have high melting points, conduct electricity when molten or aqueous, and often dissolve in water to give ions.
· Covalent molecular chlorides often have lower melting/boiling points and react/hydrolyse with water.
· Giant covalent SiO₂ has a very high melting point and does not react with water.
· If a chloride gives steamy white fumes with water, infer hydrolysis producing HCl and likely covalent bonding.
· If an oxide reacts with acid only, it is likely basic.
· If an oxide reacts with base only, it is likely acidic.
· If an oxide/hydroxide reacts with both acid and base, it is amphoteric.

Must-know equation bank

· 4Na + O₂ → 2Na₂O.
· 2Mg + O₂ → 2MgO.
· 4Al + 3O₂ → 2Al₂O₃.
· P₄ + 5O₂ → P₄O₁₀.
· S + O₂ → SO₂.
· 2Na + Cl₂ → 2NaCl.
· Mg + Cl₂ → MgCl₂.
· 2Al + 3Cl₂ → 2AlCl₃.
· Si + 2Cl₂ → SiCl₄.
· P₄ + 10Cl₂ → 4PCl₅.
· 2Na + 2H₂O → 2NaOH + H₂.
· Mg + H₂O(g) → MgO + H₂.
· Na₂O + H₂O → 2NaOH.
· MgO + H₂O → Mg(OH)₂.
· P₄O₁₀ + 6H₂O → 4H₃PO₄.
· SO₂ + H₂O → H₂SO₃.
· SO₃ + H₂O → H₂SO₄.
· SiCl₄ + 2H₂O → SiO₂ + 4HCl.
· PCl₅ + 4H₂O → H₃PO₄ + 5HCl.

Common exam traps

· Do not say all Period 3 oxides react with water: Al₂O₃ and SiO₂ do not.
· Do not call Al₂O₃ only basic: it is amphoteric.
· Do not forget that SiO₂ is acidic even though it does not react with water.
· Do not confuse dissolving with hydrolysis: NaCl dissolves, but SiCl₄ hydrolyses.
· Do not omit state/observation clues in explanations: white fumes = HCl, white precipitate = SiO₂.
· Do not explain trends using only “reactivity”; exam answers need bonding, electronegativity and structure.