Question 4Write the chemical formula of the following.(a) Magnesium chloride(b) Calcium oxide(c) Copper nitrate(d) Aluminium chloride(e) Calcium carbonate

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(a) Magnesium chlorideStep 1 — Write each symbol with its valencyMg2+↗Cl1−\text{Mg}^{2+} \phantom{\nearrow} \text{Cl}^{1-}Mg2+↗Cl1−Step 2 — Interchange the valenciesMg22 ↘↙ Cl1⇒Mg11 ↘↙ Cl2\overset{\phantom{2}{2}}{\text{Mg}} \space {\searrow}\mathllap{\swarrow} \space \overset{1}{\text{Cl}} \Rightarrow \underset{\phantom{1}{1}}{\text{Mg}} \space {\searrow}\mathllap{\swarrow} \space \underset{2}{\text{Cl}} \[0.5em]Mg22​ ↘↙ Cl1⇒11Mg​ ↘↙ 2Cl​Step 3 — Write the interchanged number, ignore equal numbers & hence the formulaTherefore, we getFormula of Magnesium chloride : MgCl2\bold{MgCl_2}MgCl2​(b) Calcium oxideStep 1 — Write each symbol with its valencyCa2+↗O2−\text{Ca}^{2+} \phantom{\nearrow} \text{O}^{2-}Ca2+↗O2−Step 2 — Interchange the valenciesCa22 ↘↙ O2⇒Ca12 ↘↙ O2\overset{\phantom{2}{2}}{\text{Ca}} \space {\searrow}\mathllap{\swarrow} \space \overset{2}{\text{O}} \Rightarrow \underset{\phantom{1}{2}}{\text{Ca}} \space {\searrow}\mathllap{\swarrow} \space \underset{2}{\text{O}} \[0.5em]Ca22 ↘↙ O2⇒12Ca​ ↘↙ 2O​Step 3 — Write the interchanged number, ignore equal numbers & hence the formulaTherefore, we getFormula of Calcium oxide : CaO\bold{CaO}CaO(c) Copper nitrateStep 1 — Write each symbol with its valencyCu2+↗NO31−\text{Cu}^{2+} \phantom{\nearrow} \text{NO}_3^{1-}Cu2+↗NO31−​Step 2 — Interchange the valenciesCu22 ↘↙ NO31⇒Cu11 ↘↙ NO32\overset{\phantom{2}{2}}{\text{Cu}} \space {\searrow}\mathllap{\swarrow} \space \overset{1}{\text{NO}_3} \Rightarrow \underset{\phantom{1}{1}}{\text{Cu}} \space {\searrow}\mathllap{\swarrow} \space \underset{2}{\text{NO}_3} \[0.5em]Cu22 ↘↙ NO3​1​⇒11Cu​ ↘↙ 2NO3​​Step 3 — Write the interchanged number, ignore equal numbers & hence the formulaTherefore, we getFormula of copper nitrate : Cu(NO3)2\bold{Cu(NO_3)_2}Cu(NO3​)2​(d) Aluminium chlorideStep 1 — Write each symbol with its valencyAl3+↗Cl1−\text{Al}^{3+} \phantom{\nearrow} \text{Cl}^{1-}Al3+↗Cl1−Step 2 — Interchange the valenciesAl23 ↘↙ 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]Al23 ↘↙ Cl1⇒11Al​ ↘↙ 3Cl​Step 3 — Write the interchanged number, ignore equal numbers & hence the formulaTherefore, we getFormula of Aluminium chloride : AlCl3\bold{AlCl_3}AlCl3​(e) Calcium carbonate –Step 1 — Write each symbol with its valencyCa2+↗CO31−\text{Ca}^{2+} \phantom{\nearrow} \text{CO}_3^{1-}Ca2+↗CO31−​Step 2 — Interchange the valenciesCa22 ↘↙ CO31⇒Ca11 ↘↙ CO32\overset{\phantom{2}{2}}{\text{Ca}} \space {\searrow}\mathllap{\swarrow} \space \overset{1}{\text{CO}_3} \Rightarrow \underset{\phantom{1}{1}}{\text{Ca}} \space {\searrow}\mathllap{\swarrow} \space \underset{2}{\text{CO}_3} \[0.5em]Ca22 ↘↙ CO3​1​⇒11Ca​ ↘↙ 2CO3​​Step 3 — Write the interchanged number, ignore equal numbers & hence the formulaTherefore, we getFormula of copper nitrate : Ca(CO3)2\bold{Ca(CO_3)_2}Ca(CO3​)2​

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  Science | Chapter 3: Atoms and MoleculesWeb Content &mdash; Quick Reference
  
    Chapter 3: Atoms and Molecules &mdash; 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 &mdash; they are frequently asked in exams.
        Focus on comparison tables, diagrams, and definitions &mdash; these are high-scoring areas.
        For numericals (if applicable), practice at le

Atoms and Molecules — Question 4

Question 4

Chapter 3: Atoms and Molecules — Quick Reference

Quick Revision Points