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Chemistry · Practice book · Senior 4 · Unit 14 of 17

Trends in chemical properties of period 3 elements and their compounds

Senior 4 Chemistry · 14 questions with answers. Try each question on paper, then tap it to see the answer.

Unit 14 Trends in chemical properties of period 3 elements and their compounds

Key unit competence: Interpret the properties of period 3 elements and their compounds to explain their chemical behaviour.

Key ideas

  • Period 3: Na, Mg, Al (metals), Si (metalloid), P, S, Cl, Ar (non-metals). Structure changes from metallic (Na, Mg, Al) to giant covalent (Si) to simple molecular (P4, S8, Cl2) to monatomic (Ar).
  • Reaction with oxygen: 4Na + O2 → 2Na2O, 2Mg + O2 → 2MgO, 4Al + 3O2 → 2Al2O3, Si + O2 → SiO2, P4 + 5O2 → P4O10, S + O2 → SO2. Maximum oxidation state rises from +1 (Na) to +6 (S in SO3) and +7 (Cl in Cl2O7).
  • Oxides: Na2O and MgO are ionic and basic; Al2O3 is amphoteric; SiO2, P4O10, SO2, SO3 are covalent and acidic. With water: Na2O gives NaOH (pH 13–14); MgO slightly alkaline (pH 9); Al2O3 and SiO2 insoluble; P4O10 gives H3PO4 (pH 2); SO3 gives H2SO4 (pH 1).
  • Chlorides: NaCl and MgCl2 are ionic; NaCl dissolves without hydrolysis (pH 7), MgCl2 slightly acidic (pH 6.5). AlCl3 (covalent, Al2Cl6 dimer), SiCl4 and PCl5 hydrolyse in water giving HCl (pH 1–3): SiCl4 + 2H2O → SiO2 + 4HCl, PCl5 + 4H2O → H3PO4 + 5HCl.
  • Reaction with water: Na reacts vigorously with cold water; Mg slowly with cold water, rapidly with steam; Al is protected by its oxide; chlorine dissolves to form HCl and HOCl.
  • Trend across the period: elements change from metallic to non-metallic; oxides and hydroxides change from basic through amphoteric to acidic; bonding in chlorides and oxides changes from ionic to covalent as electronegativity difference falls.

Common misconceptions

  • "All period 3 oxides dissolve in water." No: Al2O3 and SiO2 are insoluble in water.
  • "NaCl solution is acidic because it contains chloride." No: NaCl does not hydrolyse; its solution is neutral (pH 7).
  • "Silicon dioxide is basic because silicon is near the metals." No: SiO2 is an acidic oxide; it reacts with hot concentrated alkali.

Worked examples

Example 1. Describe the reactions of the oxides Na2O, Al2O3 and SO2 with water, acids or alkalis, to show the change in acid–base character across period 3.

Solution

Na2O (basic): Na2O + H2O → 2NaOH; reacts with acids: Na2O + 2HCl → 2NaCl + H2O.

Al2O3 (amphoteric): Al2O3 + 6HCl → 2AlCl3 + 3H2O and Al2O3 + 2NaOH + 3H2O → 2Na[Al(OH)4].

SO2 (acidic): SO2 + H2O → H2SO3; SO2 + 2NaOH → Na2SO3 + H2O.

Example 2. Explain the change in the pH of solutions formed when the chlorides of period 3 are added to water.

Solution

NaCl: dissolves without reaction, pH 7.

MgCl2: slight hydrolysis, pH about 6.5.

AlCl3: hydrolyses, [Al(H2O)6]3+ releases H+, pH about 3.

SiCl4 and PCl5: complete hydrolysis to HCl, pH 1–2.

The change reflects the change from ionic to covalent chlorides.

Practice questions

  1. Write the equation for magnesium burning in oxygen.

    Show answer

    2Mg + O2 → 2MgO.

    Common mistake: Writing MgO2.

  2. Classify Na2O, Al2O3 and P4O10 by acid–base character.

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    Basic, amphoteric, acidic.

    Common mistake: Calling Al2O3 acidic.

  3. Write the equation for SO2 reacting with water.

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    SO2 + H2O → H2SO3.

    Common mistake: Writing H2SO4.

  4. Explain why NaCl has a high melting point but SiCl4 is a liquid.

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    NaCl is giant ionic; SiCl4 is simple molecular with weak forces.

    Common mistake: Saying Si is heavier.

  5. Give the formula and pH of the solution formed by MgO in water.

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    Mg(OH)2; pH about 9 (slightly soluble).

    Common mistake: Saying pH 14.

  6. Why does aluminium chloride give an acidic solution?

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    Al3+ hydrolyses water (or covalent AlCl3 reacts) to release H+/HCl.

    Common mistake: Saying it contains sulfuric acid.

  7. Write the equation for silicon burning in oxygen.

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    Si + O2 → SiO2.

    Common mistake: Writing SiO.

  8. State the trend in electronegativity across period 3 and its effect on bonding.

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    Increases; bonds with oxygen/chlorine change from ionic to covalent.

    Common mistake: Saying it decreases.

  9. Which period 3 oxides are insoluble in water?

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    Al2O3 and SiO2.

    Common mistake: Including MgO as completely insoluble.

  10. Write the equation for Na2O with dilute HCl.

    Show answer

    Na2O + 2HCl → 2NaCl + H2O.

    Common mistake: Writing hydrogen as a product.

Matching and drag-and-drop

  1. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. The chloride that dissolves in water without hydrolysis is:a. Sodium
    2. Which period 3 element reacts most vigorously with cold water?b. NaCl
    3. Which period 3 element has a giant covalent structure?c. Silicon
    4. Which period 3 chloride exists as a dimer in the vapour?d. Aluminium chloride
    Show answer

    1 → b (NaCl) · 2 → a (Sodium) · 3 → c (Silicon) · 4 → d (Aluminium chloride)

    • It just dissociates into ions.
    • Most reactive metal in the period.
    • Each Si bonded to four others.
    • Al2Cl6.

  2. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    basic  ·  SiO2 and HCl  ·  ionic to covalent  ·  neutral
    1. Bonding in oxides across period 3 changes from .
    2. Magnesium oxide is .
    3. Silicon tetrachloride reacts with water to produce .
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    1: ionic to covalent · 2: basic · 3: SiO2 and HCl

    • Electronegativity difference decreases.
    • Ionic metal oxide.
    • Hydrolysis gives acidic solution.

  3. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. Across period 3, the elements change from:a. HCl and HOCl
    2. Which period 3 oxide is amphoteric?b. sulfuric acid
    3. Chlorine reacts with water to form:c. metals to non-metals
    4. SO3 dissolves in water to form:d. Al2O3
    Show answer

    1 → c (metals to non-metals) · 2 → d (Al2O3) · 3 → a (HCl and HOCl) · 4 → b (sulfuric acid)

    • Na is a metal; Cl is a non-metal.
    • Reacts with acids and alkalis.
    • Cl2 + H2O → HCl + HOCl.
    • SO3 + H2O → H2SO4.

  4. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    +6  ·  Mg2+ slightly hydrolyses water  ·  phosphoric acid  ·  +7
    1. The highest oxidation state of sulfur in its oxides is .
    2. P4O10 reacts with water to form .
    3. Magnesium chloride solution is slightly acidic (pH 6.5) because .
    Show answer

    1: +6 · 2: phosphoric acid · 3: Mg2+ slightly hydrolyses water

    • In SO3.
    • P4O10 + 6H2O → 4H3PO4.
    • Small, charged ion polarises water molecules.

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