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Atoms and Molecules — Question 1

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Question 1

Write down the formulae of

(i) sodium oxide

(ii) aluminium chloride

(iii) sodium sulphide

(iv) magnesium hydroxide

Answer

(i) Sodium oxide:

Step 1 — Write each symbol with its valency

Na1+↗O2−\text{Na}^{1+} \phantom{\nearrow} \text{O}^{2-}

Step 2 — Interchange the valencies

Na21 ↘↙ O2⇒Na12 ↘↙ O1\overset{\phantom{2}{1}}{\text{Na}} \space {\searrow}\mathllap{\swarrow} \space \overset{2}{\text{O}} \Rightarrow \underset{\phantom{1}{2}}{\text{Na}} \space {\searrow}\mathllap{\swarrow} \space \underset{1}{\text{O}} \\[0.5em]

Step 3 — Write the interchanged number, ignore equal numbers & hence the formula

Therefore, we get

Formula of Sodium oxide : Na2O\bold{Na_2O}

(ii) Aluminium chloride

Step 1 — Write each symbol with its valency

Al3+↗Cl1−\text{Al}^{3+} \phantom{\nearrow} \text{Cl}^{1-}

Step 2 — Interchange the valencies

Al23 ↘↙ Cl1⇒Al11 ↘↙ Cl3\overset{\phantom{2}{3}}{\text{Al}} \space {\searrow}\mathllap{\swarrow} \space \overset{1}{\text{Cl}} \Rightarrow \underset{\phantom{1}{1}}{\text{Al}} \space {\searrow}\mathllap{\swarrow} \space \underset{3}{\text{Cl}} \\[0.5em]

Step 3 — Write the interchanged number, ignore equal numbers & hence the formula

Therefore, we get

Formula of Aluminium chloride : AlCl3\bold{AlCl_3}

(iii) sodium sulphide

Step 1 — Write each symbol with its valency

Na1+↗S2−\text{Na}^{1+} \phantom{\nearrow} \text{S}^{2-}

Step 2 — Interchange the valencies

Na21 ↘↙ S2⇒Na12 ↘↙ S1\overset{\phantom{2}{1}}{\text{Na}} \space {\searrow}\mathllap{\swarrow} \space \overset{2}{\text{S}} \Rightarrow \underset{\phantom{1}{2}}{\text{Na}} \space {\searrow}\mathllap{\swarrow} \space \underset{1}{\text{S}} \\[0.5em]

Step 3 — Write the interchanged number, ignore equal numbers & hence the formula

Therefore, we get

Formula of Sodium sulphide : Na2S\bold{Na_2S}

(iv) Magnesium hydroxide

Step 1 — Write each symbol with its valency

Mg2+↗OH1−\text{Mg}^{2+} \phantom{\nearrow} \text{OH}^{1-}

Step 2 — Interchange the valencies

Mg22 ↘↙ OH1⇒Mg11 ↘↙ OH2\overset{\phantom{2}{2}}{\text{Mg}} \space {\searrow}\mathllap{\swarrow} \space \overset{1}{\text{OH}} \Rightarrow \underset{\phantom{1}{1}}{\text{Mg}} \space {\searrow}\mathllap{\swarrow} \space \underset{2}{\text{OH}} \\[0.5em]

Step 3 — Write the interchanged number, ignore equal numbers & hence the formula

Therefore, we get

Formula of Magnesium hydroxide : Mg(OH)2\bold{Mg(OH)_2}

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Science | Chapter 3: Atoms and MoleculesWeb Content — Quick Reference

Chapter 3: Atoms and Molecules — Quick Reference

Dalton theory atomic mass molecular mass mole concept chemical formula valency Avogadro number

Quick Revision Points

  • Law of conservation of mass: total mass unchanged in a reaction
  • Law of constant proportions: elements in fixed mass ratio in a compound
  • 1 u = 1/12 mass of C-12 = 1.66 × 10⁻²⁴ g
  • Molecular mass = sum of atomic masses of all atoms in molecule
  • 1 mole = 6.022 × 10²³ particles (Avogadro's number)
  • Moles = mass / molar mass; Particles = moles × Nₐ
  • Criss-cross method for writing chemical formulae using valencies
  • Polyatomic ions: NH₄⁺, OH⁻, SO₄²⁻, NO₃⁻, CO₃²⁻, PO₄³⁻
Exam Tips for Chapter 3
  • Read the detailed chapter notes for complete coverage of all NCERT topics.
  • Practice all NCERT in-text and back exercise questions — they are frequently asked in exams.
  • Focus on comparison tables, diagrams, and definitions — these are high-scoring areas.
  • For numericals (if applicable), practice at least 20 problems of varying difficulty.
  • Refer to the practice question bank (200+ questions) for thorough preparation.
Related Resources
  • Detailed Notes: ch03-atoms-and-molecules.html
  • Practice Questions: 100+ questions with answers in 05-practice-questions/
  • Chapter Test: 30-mark test paper in 06-tests/chapter-tests-30marks/
  • Formula Sheet: Complete formula reference in 03-teacher-aid/formula-sheet.html
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