Class 9 Science Chapter 9: Atomic Foundations of Matter — Solutions

These Class 9 Science Exploration Chapter 9 solutions cover Atomic Foundations of Matter from NCERT’s Exploration, first edition April 2026 (printed pages 162–183).

Class 9 Science Exploration Chapter 9 Solutions – Overview

Chapter 9 of Exploration, Atomic Foundations of Matter, explains how atoms combine to form compounds. It covers ions and valency, the formation of ionic bonds (transfer of electrons) and covalent bonds (sharing of electrons), how to write chemical formulae, how to calculate formula unit mass, and the law of conservation of mass and law of constant proportions. The worked answers include electron-dot models, balanced formulae and mass calculations, with notes where the textbook uses a simplified model.

Key Concepts & Definitions

Valency & Formula-Writing Rules

Think It Over — Answers

Are water samples from different sources chemically identical?

What is the difference between O and O₂?

Why does dissolved salt conduct electricity but sugar does not?

Activities and Pause and Ponder

Activities 9.1–9.3: Accounting for mass

For salt dissolving in water, the total mass is unchanged within measurement uncertainty if no material is lost. In the open vinegar/baking-soda setup, escaping CO₂ lowers the mass retained on the balance. In the balloon setup, the gas remains part of the weighed system. Small changes in apparent readings can also arise from leaks, splashes and air buoyancy as the balloon expands; record observations rather than inventing identical readings.

$$\mathrm{CH_3COOH(aq)}+\mathrm{NaHCO_3(s)}\longrightarrow$$$$\mathrm{CH_3COONa(aq)}+\mathrm{H_2O(l)}+\mathrm{CO_2(g)}$$$$\mathrm{Na_2SO_4(aq)}+\mathrm{BaCl_2(aq)}\longrightarrow$$$$\mathrm{BaSO_4(s)}+2\mathrm{NaCl(aq)}$$

The second reaction produces a white precipitate. Keep both vessels in the mass accounting; soluble barium salts require teacher-managed laboratory handling and disposal. A hydrogen-producing zinc/acid investigation also needs supervision, controlled quantities and gas capture without dangerous pressure buildup.

Activity 9.4: Solubility and conductivity

Record actual observations. Sodium chloride and copper sulfate form conducting aqueous solutions; sugar dissolves but remains mainly molecular. Camphor and naphthalene have low water solubility. Do not generalise that all covalent compounds are insoluble: sugar is a counterexample. Molecular substances that form ions in water can conduct too. Use an approved low-voltage tester under supervision; the textbook’s petrol/kerosene tests need laboratory controls, not unsupervised home trials.

Pause and Ponder: Answers 1–7

1. No law is violated when ethanol leaves no residue: oxygen enters and gaseous products leave. Include the surroundings in the mass balance.
2. 20 + 160 = 180 g water, with complete reaction and no loss.
3. Oxygen mass:

$$m_{\mathrm O}=20\left(\frac{60}{40}\right)\,\mathrm g=30\,\mathrm g$$

4. Oxygen needed:

$$m_{\mathrm O}=9\left(\frac43\right)\,\mathrm g=12\,\mathrm g$$

5. A specified pure compound has fixed composition; mixture proportions may vary.
6. 4:1 and 8:2 simplify to the same 4:1 mass ratio, consistent with constant proportions for the same oxide.
7. Option (iii): A is true; R is false because Dalton’s simple whole-number rule concerns numbers of atoms, not a universal whole-number mass ratio.

Pause and Ponder: Answers 8–18

8. N₂ has a triple bond and one lone pair on each N.
9. Each F has seven valence electrons; sharing one pair forms F–F, with three lone pairs on each F.
10. CO₂: O=C=O with two lone pairs on each O. H₂S: two S–H single bonds and two lone pairs on S. NH₃: three N–H single bonds and one lone pair on N. These Lewis structures count valence electrons; they do not imply all these molecules are linear.
11. Neon has the complete 2, 8 distribution and is largely unreactive under ordinary conditions.
12. Oxygen gains two electrons to form O²⁻.
13. Cl⁻; one Mg²⁺ and two Cl⁻ ions give MgCl₂.
14. K (2, 8, 8, 1) loses one electron; Ca (2, 8, 8, 2) loses two. KCl has a 1:1 ion ratio; CaCl₂ has a 1:2 ratio.
15. Two Na atoms each lose one electron; sulfur gains both to form S²⁻. Two Na⁺ balance one S²⁻ in Na₂S.
16. Carbon dioxide; nitrogen dioxide; sulfur hexafluoride; phosphorus trichloride.
17. NaHCO₃; SO₂; FeCl₃; Cu₂O.
18. Fe(OH)₃; K₂CO₃.

Pause and Ponder: Answers 19–24

19. The observation supports ionic bonding, with mobile ions in solution; some molecular substances also ionise in water, so conductivity alone does not uniquely prove solid-state bonding.
20. M is magnesium: MgO, mainly ionic in this introductory model. The small dissolved/reacted fraction provides ions, so conductivity is limited by low solubility; do not claim the whole sample dissolves.
21. Nitric acid:

$$m(\mathrm{HNO_3})=[1+14+3(16)]\,\mathrm u=63\,\mathrm u$$

22. Methane:

$$m(\mathrm{CH_4})=[12+4(1)]\,\mathrm u=16\,\mathrm u$$

23. Potassium chloride:

$$m(\mathrm{KCl})=(39+35.5)\,\mathrm u=74.5\,\mathrm u$$

24. Magnesium hydroxide:

$$m(\mathrm{Mg(OH)_2})=[24+2(16+1)]\,\mathrm u=58\,\mathrm u$$

Journey Beyond: Sample approaches

For an atom/ion bar graph, use Na: 11→10 electrons, Mg: 12→10 and Cl: 17→18. For a charge-card game, combine two Al³⁺ and three SO₄²⁻ cards into Al₂(SO₄)₃. Compare the composition of purified water by teacher-led analysis or trustworthy supplied data; natural water samples contain impurities and do not justify a direct equality claim without purification.

Class 9 Science Exploration Chapter 9 Solutions — Revise, Reflect, Refine

1. Element A has one electron in its third shell; element B has six electrons in its second shell. (i) How many electrons does A tend to give or take to become stable? (ii) What kind of ion would it form? (iii) How many electrons does B tend to give or take to become stable? (iv) What kind of ion would it form? (v) If A and B combine, what kind of bond would form? (vi) What would be the formula of the compound?

2. Element X has six electrons in its outer shell and forms a diatomic molecule. (i) Why would that be so? (ii) What kind of bond would it form? (iii) Draw the structure of the molecule it would form. (iv) Another element Y has two electrons in its second shell. Draw the structure of the molecule X would form with Y.

3. You want an ionic compound with total positive charge 6+ and total negative charge 6−. Which combination gives the correct number of ions? (i) 2 Al³⁺ and 3 Cl⁻ (ii) 3 Mg²⁺ and 1 PO₄³⁻ (iii) 2 Fe³⁺ and 3 O²⁻ (iv) 3 Ca²⁺ and 2 SO₄²⁻

4. Choose the correct statement(s) and correct the false ones. (i) Elements are made up of molecules and compounds are made up of atoms. (ii) The molecule of a compound is always made up of two or more atoms of the same kind. (iii) One molecule of nitrogen gas contains three nitrogen atoms. (iv) Water is made of two hydrogen atoms covalently bonded with one oxygen atom.

5. Write the chemical formulae for: (i) Aluminium nitrate (ii) Calcium oxide (iii) Ferric oxide

6. Write the formulae of the compounds formed from these pairs of ions. (i) Ca²⁺ and Br⁻ (ii) Al³⁺ and CO₃²⁻ (iii) K⁺ and SO₄²⁻ (iv) NH₄⁺ and Cl⁻

7. Which option in Fig. 9.18 correctly represents the Cl⁻ ion (atomic number of chlorine = 17)?

8. Determine the formula unit mass of: (i) Ammonium nitrate (NH₄NO₃) (ii) Phosphoric acid (H₃PO₄) (iii) Sodium hydrogencarbonate (NaHCO₃)

9. Write the formulae of the compounds formed by the reaction of: (i) Magnesium and nitrogen (ii) Lithium and nitrogen (iii) Sodium and sulfur (iv) Aluminium and oxygen

10. Complete Table 9.3 with the formulae of the compounds (LiNO₃ is given as an example).

11. 5.3 g of sodium carbonate and 6.0 g of acetic acid react to produce 2.2 g of carbon dioxide, 0.9 g of water and 8.2 g of sodium acetate. Verify the law of conservation of mass.

12. A species has 11 protons, 12 neutrons and 10 electrons. (i) What is its atomic number and mass number? (ii) Is it neutral, a cation or an anion? Explain. (iii) Write its electronic configuration. (iv) Name the species.

13. Two elements have configurations A: 2, 8, 5 and B: 2, 8, 7. (i) Which element is more reactive? (ii) Will A and B form ionic or covalent bonds? Explain using electron transfer or sharing. (iii) Predict the formula of the compound they form.

14. Assertion (A): Copper sulfate conducts electricity in the molten state but not in the solid state. Reason (R): Copper and sulfate ions are fixed in the lattice in the molten state, while in the solid state they can move freely. Choose the correct option: (i) Both A and R are true, and R is the correct explanation of A. (ii) Both A and R are true, but R is not the correct explanation of A. (iii) A is true, but R is false. (iv) A is false, but R is true.

15. The species ²⁷Al, ⁸⁰Br⁻ and ²⁰¹Hg²⁺ have 13, 35 and 80 protons respectively. How many electrons and neutrons do they have?

Common Mistakes to Avoid

Extra Practice Questions

Very Short Answer Type Questions

Q1. What is the valency of oxygen?

Q2. Name the bond formed by sharing of electrons.

Q3. Write the formula of magnesium chloride.

Short Answer Type Questions

Q1. Differentiate between an ionic bond and a covalent bond.

Q2. Calculate the formula unit mass of calcium carbonate (CaCO₃). (Ca = 40)

Long Answer Type Question

Q1. Explain the formation of sodium chloride (NaCl) by the transfer of electrons.

MCQs & Assertion–Reason

For each Assertion–Reason question, choose: (A) Both true and the Reason correctly explains the Assertion; (B) Both true but the Reason is not the correct explanation; (C) Assertion true, Reason false; (D) Assertion false, Reason true.

Quick Revision Summary

Real-life Applications

Frequently Asked Questions