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ICSE Class 7 Chemistry Question 9 of 16

Atomic Structure — Question 14

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Question 14

State what is meant by the term – chemical formula. Give the chemical formula of –

(a) sodium hydroxide

(b) sodium nitrate

(c) sodium bicarbonate

(d) sodium sulphite

(e) magnesium nitrate

(f) ammonium sulphate

(g) carbonic acid

(h) calcium phosphate

Answer

Chemical formula is the representation of a substance by means of symbols. It denotes in a compound, the number of atoms of each element present.

(a) Sodium hydroxide

Na1+↗OH1−Na11 ↘↙ OH1⇒Na1 ↘↙ OH1\text{Na}^{1+} \phantom{\nearrow} \text{OH}^{1-} \\[0.5em] \overset{\phantom{1}{1}}{\text{Na}} \space {\searrow}\mathllap{\swarrow} \space \overset{1}{\text{OH}} \Rightarrow \underset{\phantom{}1{}}{\text{Na}} \space {\searrow}\mathllap{\swarrow} \space \underset{1}{\text{OH}} \\[0.5em]

So, we get the formula as NaOH\bold{Na}\bold{OH}

(b) Sodium nitrate

Na1+↗NO31−Na11 ↘↙ NO31⇒Na1 ↘↙ NO31\text{Na}^{1+} \phantom{\nearrow} \text{NO}_3^{1-} \\[0.5em] \overset{\phantom{1}{1}}{\text{Na}} \space {\searrow}\mathllap{\swarrow} \space \overset{1}{\text{NO}_3} \Rightarrow \underset{\phantom{}1{}}{\text{Na}} \space {\searrow}\mathllap{\swarrow} \space \underset{1}{\text{NO}_3} \\[0.5em]

So, we get the formula as NaNO3\bold{Na}\bold{NO}_3

(c) Sodium bicarbonate

Na1+↗HCO31−Na11 ↘↙ HCO31⇒Na1 ↘↙ HCO31\text{Na}^{1+} \phantom{\nearrow} \text{HCO}_3^{1-} \\[0.5em] \overset{\phantom{1}{1}}{\text{Na}} \space {\searrow}\mathllap{\swarrow} \space \overset{1}{\text{HCO}_3} \Rightarrow \underset{\phantom{}1{}}{\text{Na}} \space {\searrow}\mathllap{\swarrow} \space \underset{1}{\text{HCO}_3} \\[0.5em]

So, we get the formula as NaHCO3\bold{Na}\bold{HCO}_3

(d) Sodium sulphite

Na1+↗SO32−Na11 ↘↙ SO32⇒Na2 ↘↙ SO31\text{Na}^{1+} \phantom{\nearrow} \text{SO}_3^{2-} \\[0.5em] \overset{\phantom{1}{1}}{\text{Na}} \space {\searrow}\mathllap{\swarrow} \space \overset{2}{\text{SO}_3} \Rightarrow \underset{\phantom{}2{}}{\text{Na}} \space {\searrow}\mathllap{\swarrow} \space \underset{1}{\text{SO}_3} \\[0.5em]

So, we get the formula as Na2SO3\bold{Na}_2\bold{SO}_3

(e) Magnesium nitrate

Mg2+↗NO31−Mg12 ↘↙ NO31⇒Mg21 ↘↙ NO32\text{Mg}^{2+} \phantom{\nearrow} \text{NO}_3^{1-} \\[0.5em] \overset{\phantom{1}{2}}{\text{Mg}} \space {\searrow}\mathllap{\swarrow} \space \overset{1}{\text{NO}_3} \Rightarrow \underset{\phantom{2}{1}}{\text{Mg}} \space {\searrow}\mathllap{\swarrow} \space \underset{2}{\text{NO}_3} \\[0.5em]

So, we get the formula as Mg(NO3)2\bold{Mg}\bold{(NO}_3)_2

(f) Ammonium sulphate

NH41+↗SO42−NH411 ↘↙ SO42⇒NH422 ↘↙ SO41\text{NH}_4^{1+} \phantom{\nearrow} \text{SO}_4^{2-} \\[0.5em] \overset{\phantom{1}{1}}{\text{NH}_4} \space {\searrow}\mathllap{\swarrow} \space \overset{2}{\text{SO}_4} \Rightarrow \underset{\phantom{2}{2}}{\text{NH}_4} \space {\searrow}\mathllap{\swarrow} \space \underset{1}{\text{SO}_4} \\[0.5em]

So, we get the formula as (NH4)2SO4\bold{(NH}_4)_2\bold{SO}_4

(g) Carbonic acid

H1+↗CO32−H11 ↘↙ CO32⇒H22 ↘↙ CO31\text{H}^{1+} \phantom{\nearrow} \text{CO}_3^{2-} \\[0.5em] \overset{\phantom{1}{1}}{\text{H}} \space {\searrow}\mathllap{\swarrow} \space \overset{2}{\text{CO}_3} \Rightarrow \underset{\phantom{2}{2}}{\text{H}} \space {\searrow}\mathllap{\swarrow} \space \underset{1}{\text{CO}_3} \\[0.5em] So, we get the formula as H2CO3\bold{H}_2\bold{CO}_3

(h) Calcium phosphate

Ca2+↗PO43−Ca12 ↘↙ PO43⇒Ca23 ↘↙ PO42\text{Ca}^{2+} \phantom{\nearrow} \text{PO}_4^{3-} \\[0.5em] \overset{\phantom{1}{2}}{\text{Ca}} \space {\searrow}\mathllap{\swarrow} \space \overset{3}{\text{PO}_4} \Rightarrow \underset{\phantom{2}{3}}{\text{Ca}} \space {\searrow}\mathllap{\swarrow} \space \underset{2}{\text{PO}_4} \\[0.5em]

So, we get the formula as Ca3(PO4)2\bold{Ca}_\bold{3}\bold{(PO_4)_2}

ICSE Class VII Chemistry Chapter 3 15 Marks

Atomic Structure — Quick Study Guide

Atoms are the building blocks of all matter. Understanding atomic structure is essential for all of chemistry.

Key Definitions

  • Atom: Smallest particle of an element with its chemical properties
  • Molecule: Group of two or more atoms bonded together
  • Element: Pure substance with only one type of atom
  • Compound: Two or more different elements chemically combined
  • Atomic number (Z): Number of protons = Number of electrons
  • Mass number (A): Protons + Neutrons
  • Valency: Combining capacity of an element
  • Isotopes: Same element, different mass numbers

Electronic Configuration Rules

K shell = max 2 electrons, L shell = max 8, M shell = max 8 (for first 20 elements). Fill inner shells first.

Exam Tips

  • Memorise symbols of first 20 elements
  • Practice calculating protons, neutrons, electrons from Z and A
  • Practice drawing Bohr diagrams
  • Valency: if outermost ≤ 4, valency = outermost; if > 4, valency = 8 - outermost

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