Class 9 Science Exploration Chapter 9 Solutions (NCERT 2026–27) – Atomic Foundations of Matter

These Class 9 Science Exploration Chapter 9 solutions cover Atomic Foundations of Matter from the new NCF-2023 textbook (2026–27).

Class: 9 Subject: Science Book: Exploration Chapter: 9 Exercise: Revise, Reflect, Refine (15 Qs) Session: 2026–27
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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. These Class 9 Science Exploration Chapter 9 solutions answer every textbook question step by step.

Key Concepts & Definitions

Ion: a charged atom — a cation (positive, formed by losing electrons) or an anion (negative, formed by gaining electrons).

Valency: the combining capacity of an atom = the number of electrons it gains, loses or shares to complete its octet.

Ionic bond: formed by the transfer of electrons from a metal to a non-metal (e.g., Na+Cl−).

Covalent bond: formed by the sharing of electron pairs between non-metals (e.g., O2, H2O).

Formula unit mass: the sum of the atomic masses of all the atoms in a formula unit.

Law of conservation of mass: mass is neither created nor destroyed in a chemical reaction.

Valency & Formula-Writing Rules

Step 1: write the symbols of the ions with their charges (valencies).

Step 2: “cross over” — the numerical value of each ion’s charge becomes the subscript of the other ion.

Step 3: simplify the ratio; enclose polyatomic ions in brackets when more than one is needed.

Common ions: Na+, K+, NH4+; Ca2+, Mg2+, Cu2+; Al3+, Fe3+; Cl−, NO3−; O2−, SO42−, CO32−; PO43−.

“Think It Over” — Answers

Water can be broken down into hydrogen and oxygen. Are all samples of water identical?

ANSWERYes. Every pure sample of water has the same fixed composition — hydrogen and oxygen always combine in the same ratio (2 : 1 by atoms, about 1 : 8 by mass). This is the law of constant proportions.

Oxygen is sometimes found as O2 and sometimes as O3 (ozone). Why?

ANSWERThe same element can exist in different molecular forms (allotropes). In O2 two oxygen atoms join, while in ozone (O3) three oxygen atoms join — the atoms are identical, only the number combined in a molecule differs.

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?

ANSWER A has the configuration 2, 8, 1 (like sodium) and B has 2, 6 (like oxygen). (i) A tends to give 1 electron. (ii) A forms a cation, A+. (iii) B tends to take 2 electrons. (iv) B forms an anion, B2−. (v) A (metal) gives electrons and B (non-metal) takes them, so they form an ionic bond. (vi) Crossing over the charges (A+, B2−) gives the formula A2B.

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.

ANSWER (i) X (like oxygen) needs 2 more electrons to complete its octet; a single atom cannot, so two X atoms share two pairs of electrons (a double bond) and each completes its octet, forming X2. (ii) A covalent bond (a double covalent bond). (iii) Structure: X=X — two atoms joined by a double bond (two shared pairs), each X also carrying two lone pairs (like O2). (iv) Y has the configuration 2, 2, so it has 2 outer electrons to give while X needs 2. They combine in a 1 : 1 ratio (YX): Y transfers its two electrons to X, giving Y2+ and X2− held by an ionic bond (structure Y2+ X2−).

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−

ANSWER Correct option: (iii) — 2 Fe3+ gives +6 and 3 O2− gives −6, so both add up to 6. (i) gives +6 and only −3;(ii) gives +6 and only −3;(iv) gives +6 and only −4 — so they are not balanced at 6+ and 6−.

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.

ANSWER (i) False. Correct: both elements and compounds are made of atoms — an element has atoms of one kind, a compound has atoms of two or more different kinds. (ii) False. Correct: a molecule of a compound is made of two or more atoms of different kinds. (iii) False. Correct: nitrogen gas is diatomic — one molecule has two nitrogen atoms (N2). (iv) True — water (H2O) has two hydrogen atoms covalently bonded to one oxygen atom.

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

ANSWER (i) Aluminium nitrate: Al3+ and NO3− → Al(NO3)3. (ii) Calcium oxide: Ca2+ and O2− → CaO. (iii) Ferric oxide: Fe3+ and O2− → Fe2O3.

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−

ANSWER (i) CaBr2. (ii) Al2(CO3)3. (iii) K2SO4. (iv) NH4Cl.

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

ANSWER Correct option: (ii). A neutral chlorine atom has 17 electrons (2, 8, 7). On gaining 1 electron it becomes Cl− with 18 electrons arranged 2, 8, 8, while the nucleus still has 17 protons — giving a net charge of −1. Option (ii) is the one that shows 17 protons in the nucleus with the 2, 8, 8 electron arrangement (a complete octet). The option showing 2, 8, 7 is the neutral atom, not the ion.

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

ANSWER (Atomic masses: H = 1, C = 12, N = 14, O = 16, Na = 23, P = 31.) (i) NH4NO3 = 2(14) + 4(1) + 3(16) = 28 + 4 + 48 = 80 u. (ii) H3PO4 = 3(1) + 31 + 4(16) = 3 + 31 + 64 = 98 u. (iii) NaHCO3 = 23 + 1 + 12 + 3(16) = 23 + 1 + 12 + 48 = 84 u.

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

ANSWER (i) Mg2+ and N3− → Mg3N2. (ii) Li+ and N3− → Li3N. (iii) Na+ and S2− → Na2S. (iv) Al3+ and O2− → Al2O3.

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

ANSWER
Cation ↓ / Anion →NO3−SO42−PO43−
NH4+NH4NO3(NH4)2SO4(NH4)3PO4
Li+LiNO3Li2SO4Li3PO4
Al3+Al(NO3)3Al2(SO4)3AlPO4
Cu2+Cu(NO3)2CuSO4Cu3(PO4)2

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.

ANSWER Total mass of reactants = 5.3 + 6.0 = 11.3 g. Total mass of products = 2.2 + 0.9 + 8.2 = 11.3 g. Reactant mass = product mass (11.3 g = 11.3 g), so the law of conservation of mass is verified.

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.

ANSWER (i) Atomic number = protons = 11; mass number = protons + neutrons = 11 + 12 = 23. (ii) Protons (11) are more than electrons (10), so there is a net positive charge — it is a cation (charge +1). (iii) With 10 electrons the configuration is 2, 8. (iv) It is the sodium ion, Na+.

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.

ANSWER (i) B is more reactive. B (7 valence electrons) needs only 1 electron to complete its octet, while A (5 valence electrons) needs 3 — B is closer to a stable configuration, so it is more reactive. (ii) Both A and B are non-metals, so they complete their octets by sharing electrons — they form covalent bonds. (iii) A needs 3 electrons (valency 3) and B needs 1 (valency 1), so one A atom shares with three B atoms — formula AB3.

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.

ANSWER Correct option: (iii) — A is true, but R is false. A is true: molten copper sulfate conducts because its ions are free to move. R is false — it states the opposite; the ions are fixed in the solid lattice and free to move in the molten state.

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

ANSWER 27Al (neutral): electrons = 13; neutrons = 27 − 13 = 14. 80Br− (gained 1 electron): electrons = 35 + 1 = 36; neutrons = 80 − 35 = 45. 201Hg2+ (lost 2 electrons): electrons = 80 − 2 = 78; neutrons = 201 − 80 = 121.

Common Mistakes to Avoid

Watch out for these

  • Forgetting to balance the total positive and negative charges when writing a formula.
  • Not using brackets for polyatomic ions when more than one is needed, e.g., Al(NO3)3, not AlNO33.
  • Mixing up cation (loses electrons, positive) and anion (gains electrons, negative).
  • Thinking metals share electrons — metals usually transfer electrons (ionic), non-metals share (covalent).
  • Writing nitrogen or oxygen as single atoms — they are diatomic (N2, O2).
  • Forgetting that an ionic solid conducts only when molten or dissolved (ions free to move), not as a solid.

Extra Practice Questions

Very Short Answer Type Questions

Q1. What is the valency of oxygen?

ANSWER2.

Q2. Name the bond formed by sharing of electrons.

ANSWERA covalent bond.

Q3. Write the formula of magnesium chloride.

ANSWERMgCl2.

Short Answer Type Questions

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

ANSWERAn ionic bond forms by the transfer of electrons (usually between a metal and a non-metal) giving charged ions; a covalent bond forms by the sharing of electron pairs between non-metals. Ionic compounds conduct when molten/dissolved; most covalent compounds do not.

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

ANSWERCaCO3 = 40 + 12 + 3(16) = 40 + 12 + 48 = 100 u.

Long Answer Type Question

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

ANSWER Sodium (2, 8, 1) has 1 electron in its outer shell; it loses this electron to become Na+ with a stable 2, 8 configuration. Chlorine (2, 8, 7) needs 1 electron; it gains the electron lost by sodium to become Cl− with a stable 2, 8, 8 configuration. The oppositely charged Na+ and Cl− ions attract each other strongly, forming the ionic compound NaCl.

MCQs & Assertion–Reason

1. A positively charged ion is called a(n):

(a) anion    (b) cation    (c) molecule    (d) atom

2. The bond formed by transfer of electrons is:

(a) covalent    (b) ionic    (c) metallic    (d) hydrogen

3. The formula of aluminium oxide is:

(a) AlO    (b) Al2O3    (c) Al3O2    (d) AlO3

4. The formula unit mass of NaCl is (Na = 23, Cl = 35.5):

(a) 58.5 u    (b) 48.5 u    (c) 68.5 u    (d) 35.5 u

5. Nitrogen gas exists as:

(a) N    (b) N2    (c) N3    (d) N4

6. The valency of the ammonium ion (NH4+) is:

(a) 1    (b) 2    (c) 3    (d) 4

7. Which compound is held together by covalent bonds?

(a) NaCl    (b) MgO    (c) H2O    (d) CaCl2

8. Ionic compounds conduct electricity when:

(a) solid    (b) molten or dissolved    (c) frozen    (d) never

9. The number of electrons in a Cl− ion (Z = 17) is:

(a) 16    (b) 17    (c) 18    (d) 19

10. The law of conservation of mass was given by:

(a) Dalton    (b) Lavoisier    (c) Bohr    (d) Rutherford

Answer key: 1-(b), 2-(b), 3-(b), 4-(a), 5-(b), 6-(a), 7-(c), 8-(b), 9-(c), 10-(b).

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.

A-R 1. Assertion: Sodium forms a cation by losing one electron.

Reason: It then attains the stable configuration 2, 8.

A-R 2. Assertion: Ionic compounds conduct electricity in the molten state.

Reason: In the molten state the ions are free to move.

A-R 3. Assertion: The total mass is conserved in a chemical reaction.

Reason: Atoms are only rearranged, not created or destroyed.

A-R 4. Assertion: Oxygen gas is monatomic.

Reason: Each oxygen atom already has a complete octet.

A-R 5. Assertion: A covalent bond forms between two non-metals.

Reason: Non-metals complete their octets by sharing electrons.

Answer key: 1-(A), 2-(A), 3-(A), 4-(D), 5-(A).

Quick Revision Summary

  • Atoms combine to complete their octets by losing, gaining or sharing electrons.
  • Ionic bond = transfer of electrons (metal + non-metal); covalent bond = sharing (non-metal + non-metal).
  • Valency = combining capacity; write formulae by balancing total charges (cross-over method).
  • Formula unit mass = sum of atomic masses of all atoms in the formula.
  • Cation = positive (lost electrons); anion = negative (gained electrons).
  • Law of conservation of mass: mass of reactants = mass of products.

Real-life Applications

This chemistry is everywhere: common salt (NaCl) and washing soda are ionic compounds, while water, carbon dioxide and the gases we breathe are covalent; fertilisers such as ammonium nitrate and phosphoric-acid products are made using these formulae; molten and dissolved ionic compounds conduct electricity (used in electrolysis and batteries); and the law of conservation of mass is the basis of balancing every chemical equation.

How to score full marks in this chapter

Always balance the charges when writing a formula and use brackets for polyatomic ions. Decide the bond type from the elements (metal + non-metal → ionic; non-metal + non-metal → covalent). For formula unit mass, list each atom with its mass and add carefully, and remember mass is conserved in every reaction.

Frequently Asked Questions

What is Class 9 Science Exploration Chapter 9 about?

Atomic foundations of matter — how atoms combine, ionic and covalent bonds, ions and valency, writing chemical formulae, formula unit mass, and the law of conservation of mass.

What is the difference between an ionic and a covalent bond?

An ionic bond forms by transfer of electrons (metal to non-metal); a covalent bond forms by sharing of electrons between non-metals.

How do you write a chemical formula?

Write the ions with their charges, cross over the numerical value of each charge to the other ion as a subscript, and simplify, using brackets for polyatomic ions.

Are these Class 9 Science Exploration Chapter 9 solutions free?

Yes. All solutions are free and follow the official NCERT Exploration textbook for 2026–27.

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