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

Navigating through the newly revised CBSE Class 9 Science curriculum (Exploration) requires a thorough chemical and mathematical understanding of the Laws of Chemical Combination (the Law of Conservation of Mass and the Law of Definite/Constant Proportions), Dalton’s Atomic Theory, the physical distinctions between atoms and molecules, atomicity of elements, monoatomic and polyatomic ions (cations and anions), ionic (electrovalent) versus covalent chemical bonding, the step-by-step criss-cross method for writing chemical formulae, molecular mass and formula unit mass calculations, electrical conductivity in molten vs. solid ionic lattices, and quantitative mass verification in chemical reactions. Chapter 9 of Class 9 Chemistry, “Atomic Foundations of Matter”, builds the bridge between subatomic structure and chemical reactivity. It explains why pure water samples from rivers, rain, and oceans always contain hydrogen and oxygen combined in a fixed 1:8 mass ratio; details how electron transfer creates stable ionic crystals while electron sharing produces covalent molecules; provides the formula unit mass for industrial compounds like ammonium nitrate and sodium hydrogencarbonate; and proves the conservation of mass in acid-carbonate reactions. To help students master every aspect of this high-weightage chapter, this comprehensive solutions guide offers textbook-accurate, highly structured, and pedagogically sound responses strictly aligned with the latest CBSE Class 9 evaluation standards.

Every question presented in the official NCERT textbook—ranging from all introductory “Think It Over” sections and in-text “Pause and Ponder” prompts (Pages 168, 170, 172, and 176) to the complete end-of-chapter “Revise, Reflect, Refine” exercises (Questions 1 to 15 on Pages 180–183)—has been solved with exhaustive detail. Numerical problems and chemical formula derivations follow a step-by-step box format with explicit ion valencies and percentage calculations using clean plain-text symbols without raw LaTeX tags. Key scoring terms, official CBSE exam tags, and dynamic summary tables have been highlighted to ensure students secure maximum marks in their examinations.

Master Concept & Comparative Summary Tables

1. Common Cations, Anions, and Polyatomic Radicals Valency Sheet

Valency (Charge)	Monovalent Cations (+1)	Divalent Cations (+2)	Trivalent / Tetravalent Cations (+3 / +4)	Monovalent Anions (-1)	Divalent Anions (-2)	Trivalent Anions (-3)
Simple Elementary Ions	• Hydrogen: H⁺ • Sodium: Na⁺ • Potassium: K⁺ • Silver: Ag⁺ • Copper(I): Cu⁺	• Magnesium: Mg²⁺ • Calcium: Ca²⁺ • Zinc: Zn²⁺ • Iron(II): Fe²⁺ • Copper(II): Cu²⁺	• Aluminium: Al³⁺ • Iron(III): Fe³⁺ • Lead(IV): Pb⁴⁺	• Hydride: H⁻ • Chloride: Cl⁻ • Bromide: Br⁻ • Iodide: I⁻	• Oxide: O²⁻ • Sulphide: S²⁻	• Nitride: N³⁻ • Phosphide: P³⁻
Polyatomic Compound Radicals	• Ammonium: NH₄⁺ • Hydroxide: OH⁻ • Nitrate: NO₃⁻ • Hydrogen carbonate (Bicarbonate): HCO₃⁻	• Carbonate: CO₃²⁻ • Sulphate: SO₄²⁻ • Sulphite: SO₃²⁻	• Phosphate: PO₄³⁻	• Nitrite: NO₂⁻ • Permanganate: MnO₄⁻ • Acetate (Ethanoate): CH₃COO⁻	• Dichromate: Cr₂O₇²⁻ • Chromate: CrO₄²⁻	• Borate: BO₃³⁻

2. Comparison: Ionic (Electrovalent) Compounds vs. Covalent Molecular Compounds

Characteristic Feature	Ionic Compounds (e.g., Sodium Chloride, NaCl; Magnesium Oxide, MgO)	Covalent Compounds (e.g., Water, H₂O; Methane, CH₄; Oxygen, O₂)
Mechanism of Bond Formation	Complete transfer of valence electrons from a metal atom to a non-metal atom.	Mutual sharing of valence electron pairs between two non-metal atoms.
Constituent Chemical Units	Formed of positively charged cations and negatively charged anions.	Formed of neutral, discrete covalent molecules.
Intermolecular / Lattice Forces	Strong, non-directional electrostatic forces of attraction (Ionic lattice).	Weak intermolecular forces (Van der Waals forces) between neutral molecules.
Physical State at Room Temp	Hard, crystalline brittle solids.	Generally gases, liquids, or soft volatile solids.
Melting & Boiling Points	Very High (Requires huge thermal energy to break the strong ionic lattice).	Low to Moderate (Requires minimal energy to overcome weak intermolecular forces).
Electrical Conductivity	Conducts electricity in molten state or aqueous solution (free mobile ions); does not conduct in solid state.	Poor conductors / Non-conductors of electricity in all states (no free ions or mobile electrons).

NCERT In-Text Questions: “Think It Over”

Page No. 164: Think It Over (Questions 1 to 3)

Question 1 Water can be obtained from various natural sources (such as rivers, rain, wells, and seas). Are all these pure samples of water chemically identical? [Exam Favorite]

Answer: Yes, all pure samples of water are chemically identical.

• Scientific Justification (Law of Constant Proportions): French chemist Joseph Proust established the Law of Definite / Constant Proportions, which states that in a pure chemical compound, the constituent elements are always present in a strictly fixed ratio by mass, regardless of the source or method of preparation.
• In pure water (chemical formula H₂O), hydrogen and oxygen are always chemically combined in a fixed ratio of 1 : 8 by mass (2 g of Hydrogen per 16 g of Oxygen). Hence, every pure sample of water is identical in molecular composition and properties.

Question 2 Oxygen is sometimes represented as O and sometimes as O₂. What is the fundamental difference between these two chemical symbols? [Exam Favorite]

Answer: The difference lies in atomic versus molecular state and chemical stability:

• The Symbol ‘O’: Represents one single atom of oxygen. A single oxygen atom has an incomplete valence shell (2, 6) with 6 valence electrons; it is highly reactive, unstable, and cannot exist independently under normal atmospheric conditions.
• The Formula ‘O₂’: Represents one diatomic molecule of oxygen gas. It consists of two oxygen atoms chemically joined by a double covalent bond (O = O), allowing both atoms to complete their stable octets. Oxygen naturally exists in the atmosphere as O₂ molecules.

Question 3 Why does dissolved common salt (NaCl) in water conduct electricity, but dissolved sugar does not? [Exam Favorite]

Answer: Electrical conductivity in liquids requires free, mobile electric charge carriers (ions):

• Common Salt (NaCl) is an Ionic Compound: When solid sodium chloride crystals dissolve in water, the polar water molecules break the strong ionic lattice, causing NaCl to dissociate completely into free, mobile sodium cations (Na⁺) and chloride anions (Cl⁻). These free ions migrate toward opposite electrodes under an applied voltage, carrying an electric current.
• Sugar (C₁₂H₂₂O₁₁) is a Covalent Compound: When sugar dissolves in water, it breaks down into individual, neutral whole sugar molecules. Because no ions or charged particles are produced in the solution, dissolved sugar cannot conduct electricity.

NCERT In-Text Questions: “Pause and Ponder”

Page No. 168: Pause and Ponder (Question 1)

Question 1 How do atoms with incomplete outermost shells achieve stable noble gas electronic configurations? [Exam Favorite]

Answer: Atoms achieve a stable noble gas electronic configuration (an octet of 8 electrons, or a duplet of 2 electrons for hydrogen/helium) through three chemical bonding mechanisms:

1. By Losing Valence Electrons (Cation Formation): Metal atoms (with 1, 2, or 3 valence electrons) lose their outermost electrons to form positively charged ions (e.g., Na → Na⁺ + 1e⁻).
2. By Gaining Valence Electrons (Anion Formation): Non-metal atoms (with 5, 6, or 7 valence electrons) gain electrons to complete an octet, forming negatively charged ions (e.g., Cl + 1e⁻ → Cl⁻).
3. By Sharing Valence Electrons (Covalent Molecule Formation): Non-metal atoms mutually share one, two, or three pairs of valence electrons to complete their respective octets (e.g., H₂, O₂, N₂, CH₄).

Page No. 172: Pause and Ponder (Question 2)

Question 2 Explain why a sodium atom forms a positive cation (Na⁺) when it loses 1 valence electron, while a chlorine atom forms a negative anion (Cl⁻) when it gains 1 electron. [Exam Favorite]

Answer:

• Sodium Cation Formation (Na⁺):
  • A neutral Sodium atom (Z = 11) contains 11 protons (+11 charge) and 11 electrons (-11 charge).
  • To achieve a stable octet (2, 8), it loses its single 3s valence electron.
  • The resulting species has 11 protons (+11) and only 10 electrons (-10).
  • Net electric charge = +11 - 10 = +1. Hence, it forms a positively charged cation (Na⁺).
• Chloride Anion Formation (Cl⁻):
  • A neutral Chlorine atom (Z = 17) contains 17 protons (+17 charge) and 17 electrons (-17 charge).
  • To complete its octet (2, 8, 7 → 2, 8, 8), it gains 1 electron.
  • The resulting species has 17 protons (+17) and 18 electrons (-18).
  • Net electric charge = +17 - 18 = -1. Hence, it forms a negatively charged anion (Cl⁻).

Page No. 176: Pause and Ponder (Question 3)

Question 3 Explain the step-by-step ‘Criss-Cross’ method for writing the chemical formula of an ionic compound with an example. [Exam Favorite]

Answer:

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CRISS-CROSS METHOD RULES (EXAMPLE: ALUMINIUM OXIDE):
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STEP 1: Write down the chemical symbols of the constituent cation and anion.
        Cation = Al,    Anion = O

STEP 2: Write the numerical valency (charge magnitude) below each symbol.
        Symbol  :   Al        O
        Valency :    3        2

STEP 3: Criss-cross the valency numbers to the subscript of the opposing symbol.
        Al receives subscript 2, and O receives subscript 3.

STEP 4: Combine the symbols with their lowest whole-number ratio subscripts.
        Chemical Formula = Al₂O₃

RULE FOR POLYATOMIC RADICALS:
If a polyatomic radical takes a subscript greater than 1, enclose the radical 
in parentheses (e.g., Magnesium Hydroxide: Mg²⁺ and OH⁻ ===> Mg(OH)₂).
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NCERT Chapter-End Exercises: “Revise, Reflect, Refine” (Pages 180–183)

Question 1 A particular element (A) has one electron in its third shell. There is another element (B) with 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 were to combine, what kind of chemical bond would be formed? (vi) What would be the chemical formula of the compound thus formed? [Exam Favorite]

Answer:

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DEDUCTION & FORMULA DERIVATION:
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Element A (Electronic Configuration: K=2, L=8, M=1 ===> Sodium, Na, Z=11):
(i)  Element A tends to LOSE (give) 1 electron to achieve a stable octet (2, 8).
(ii) It forms a positively charged CATION (A⁺).

Element B (Electronic Configuration: K=2, L=6 ===> Oxygen, O, Z=8):
(iii) Element B tends to GAIN (take) 2 electrons to complete its octet (2, 8).
(iv)  It forms a negatively charged ANION (B²⁻).

(v)  Type of Bond: IONIC (ELECTROVALENT) BOND, formed by complete electron 
     transfer from metal A to non-metal B.

(vi) Chemical Formula Derivation (Criss-Cross):
     Symbol  :   A       B
     Valency :   1       2
     Criss-cross subscripts ===> A₂B  (e.g., Na₂O, Sodium Oxide).
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Question 2 An element X has six electrons in its outer shell and forms a diatomic molecule (X₂). (i) Why would that be so? (ii) What kind of bond would it form? (iii) Draw/describe the structure of the molecule it would form. (iv) A certain other element Y has two electrons in its second shell. Draw/describe the compound that X would form with Y. [Exam Favorite]

Answer:

• (i) Why X Forms a Diatomic Molecule (X₂): Element X has 6 valence electrons and requires 2 more electrons to complete its stable octet. By sharing 2 pairs of electrons with another identical X atom, both atoms achieve a stable octet configuration.
• (ii) Type of Bond in X₂: Double Covalent Bond (4 shared electrons).
• (iii) Molecular Structure of X₂: ::X = X:: (Each X atom shares two electron pairs, with two non-bonding lone pairs remaining on each atom, exactly like O₂).
• (iv) Compound Formed Between X and Y:
  • Element Y has 2 electrons in its outer shell (Valency = 2, e.g., Beryllium or Magnesium). It loses 2 electrons to form cation Y²⁺.
  • Element X needs 2 electrons, gaining them to form anion X²⁻.
  • They combine via electron transfer (Ionic Bond) in a 1:1 ratio to form the ionic compound YX (e.g., MgO or BeO).

Question 3 You want to design a new ionic compound where the total positive charge is +6 and the total negative charge is -6. Which of the following combinations gives the correct number of ions? [Exam Favorite]

Answer: To achieve an electrically neutral ionic compound with total positive charge +6 and total negative charge -6:

1. Combination 1 (Trivalent Cation + Divalent Anion): 2 × (Al³⁺) = +6 and 3 × (O²⁻) = -6 Formula: Al₂O₃ (Aluminium Oxide).
2. Combination 2 (Divalent Cation + Trivalent Anion): 3 × (Mg²⁺) = +6 and 2 × (N³⁻) = -6 Formula: Mg₃N₂ (Magnesium Nitride).
3. Combination 3 (Monovalent Cation + Trivalent Anion): 6 × (Na⁺) = +6 and 2 × (N³⁻) = -6 Formula: Na₃N (2 formula units total 6 Na⁺ and 2 N³⁻).

Question 4 Choose the correct statement(s) and correct the false statement(s): [Exam Favorite] (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.
  • Correction: Elements are pure substances made up of only one kind of atom (or homoatomic molecules like O₂, P₄). Compounds are substances made up of atoms of two or more different elements chemically combined in a fixed ratio.
• (ii) False.
  • Correction: A molecule of a compound is always made up of two or more atoms of different chemical elements chemically bonded together (e.g., CO₂, H₂O).
• (iii) False.
  • Correction: One molecule of nitrogen gas is diatomic (N₂) and contains two nitrogen atoms joined by a triple covalent bond.
• (iv) True.
  • Explanation: A water molecule (H₂O) consists of two hydrogen atoms bonded to one central oxygen atom via two single covalent bonds.

Question 5 Write the chemical formulae for the following compounds using the criss-cross method: (a) Magnesium chloride (b) Calcium oxide (c) Copper(II) nitrate (d) Aluminium chloride (e) Calcium carbonate [Exam Favorite]

Answer:

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CHEMICAL FORMULA DERIVATIONS:
--------------------------------------------------------------------------------
(a) Magnesium chloride:
    • Cation: Mg²⁺ (valency 2),  Anion: Cl⁻ (valency 1)
    • Criss-cross ===> Mg₁Cl₂ ===> Formula = MgCl₂

(b) Calcium oxide:
    • Cation: Ca²⁺ (valency 2),  Anion: O²⁻ (valency 2)
    • Ratio 2:2 simplifies to 1:1 ===> Formula = CaO

(c) Copper(II) nitrate:
    • Cation: Cu²⁺ (valency 2),  Anion: NO₃⁻ (valency 1)
    • Criss-cross ===> Cu₁(NO₃)₂ ===> Formula = Cu(NO₃)₂

(d) Aluminium chloride:
    • Cation: Al³⁺ (valency 3),  Anion: Cl⁻ (valency 1)
    • Criss-cross ===> Al₁Cl₃ ===> Formula = AlCl₃

(e) Calcium carbonate:
    • Cation: Ca²⁺ (valency 2),  Anion: CO₃²⁻ (valency 2)
    • Ratio 2:2 simplifies to 1:1 ===> Formula = CaCO₃
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Question 6 Write the formulae of the compounds formed from the following pairs of ions: (a) Na⁺ and SO₄²⁻ (b) NH₄⁺ and CO₃²⁻ (c) Fe³⁺ and O²⁻ (d) Al³⁺ and SO₄²⁻ [Exam Favorite]

Answer:

• (a) Na⁺ and SO₄²⁻ → Na₂SO₄ (Sodium sulphate)
• (b) NH₄⁺ and CO₃²⁻ → (NH₄)₂CO₃ (Ammonium carbonate)
• (c) Fe³⁺ and O²⁻ → Fe₂O₃ (Iron(III) oxide / Ferric oxide)
• (d) Al³⁺ and SO₄²⁻ → Al₂(SO₄)₃ (Aluminium sulphate)

Question 7 Which of the following correctly represents a chloride ion (Cl⁻, Atomic number of chlorine = 17)? [Exam Favorite]

Answer:

• Correct Representation: The structure showing 18 electrons arranged in shells as K = 2, L = 8, M = 8.
• Reasoning: A neutral chlorine atom has 17 protons and 17 electrons (configuration 2, 8, 7). To form the stable chloride anion (Cl⁻), it gains 1 electron into its M-shell, resulting in 18 electrons and achieving the stable octet electronic configuration of Argon (2, 8, 8).

Question 8 Determine the formula unit mass / molecular mass of the following substances: (i) Ammonium nitrate (NH₄NO₃), used as a nitrogen fertilizer. (ii) Phosphoric acid (H₃PO₄), used in detergents and fertilizers. (iii) Sodium hydrogencarbonate (NaHCO₃), used as baking soda. (Given atomic masses: H = 1 u, N = 14 u, O = 16 u, Na = 23 u, P = 31 u, C = 12 u). [Exam Favorite]

Answer:

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MOLECULAR / FORMULA UNIT MASS CALCULATIONS:
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(i) Ammonium Nitrate (NH₄NO₃):
    Total Atoms = 2 Nitrogen + 4 Hydrogen + 3 Oxygen
    Mass = (2 × 14 u) + (4 × 1 u) + (3 × 16 u)
    Mass = 28 + 4 + 48 = 80 u

(ii) Phosphoric Acid (H₃PO₄):
    Total Atoms = 3 Hydrogen + 1 Phosphorus + 4 Oxygen
    Mass = (3 × 1 u) + (1 × 31 u) + (4 × 16 u)
    Mass = 3 + 31 + 64 = 98 u

(iii) Sodium Hydrogencarbonate (NaHCO₃):
    Total Atoms = 1 Sodium + 1 Hydrogen + 1 Carbon + 3 Oxygen
    Mass = (1 × 23 u) + (1 × 1 u) + (1 × 12 u) + (3 × 16 u)
    Mass = 23 + 1 + 12 + 48 = 84 u

FINAL ANSWER:
• Formula Unit Mass of NH₄NO₃ = 80 u
• Molecular Mass of H₃PO₄      = 98 u
• Formula Unit Mass of NaHCO₃ = 84 u
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Question 9 State the Law of Conservation of Mass and the Law of Constant Proportions with illustrative chemical examples. [Exam Favorite]

Answer:

1. Law of Conservation of Mass (Antoine Lavoisier, 1774):
   • Statement: Mass can neither be created nor destroyed in a chemical reaction. The total mass of products is always strictly equal to the total mass of reactants.
   • Example: Heating 100 g of Calcium Carbonate (CaCO₃) produces exactly 56 g of Calcium Oxide (CaO) and 44 g of Carbon Dioxide (CO₂) (56 g + 44 g = 100 g).
2. Law of Constant / Definite Proportions (Joseph Proust, 1799):
   • Statement: In a pure chemical substance, the constituent elements are always combined together in a fixed, definite proportion by mass.
   • Example: In ammonia (NH₃), nitrogen and hydrogen are always present in the fixed mass ratio of 14 : 3 (14 g of Nitrogen per 3 g of Hydrogen).

Question 10 Complete the table by writing the chemical formulae of the compounds formed between the given cations and anions: [Exam Favorite]

Cation / Anion	Chloride (Cl⁻)	Hydroxide (OH⁻)	Nitrate (NO₃⁻)	Sulphate (SO₄²⁻)	Carbonate (CO₃²⁻)	Phosphate (PO₄³⁻)
Sodium (Na⁺)	NaCl	NaOH	NaNO₃	Na₂SO₄	Na₂CO₃	Na₃PO₄
Calcium (Ca²⁺)	CaCl₂	Ca(OH)₂	Ca(NO₃)₂	CaSO₄	CaCO₃	Ca₃(PO₄)₂
Aluminium (Al³⁺)	AlCl₃	Al(OH)₃	Al(NO₃)₃	Al₂(SO₄)₃	Al₂(CO₃)₃	AlPO₄
Ammonium (NH₄⁺)	NH₄Cl	NH₄OH	NH₄NO₃	(NH₄)₂SO₄	(NH₄)₂CO₃	(NH₄)₃PO₄

Question 11 In a reaction, 5.3 g of sodium carbonate reacted with 6.0 g of acetic acid. The products were 2.2 g of carbon dioxide, 0.9 g of water, and 8.2 g of sodium acetate. Show that these observations are in agreement with the Law of Conservation of Mass. [Exam Favorite]

Answer:

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VERIFICATION OF LAW OF CONSERVATION OF MASS:
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CHEMICAL EQUATION:
Sodium Carbonate + Acetic Acid ===> Sodium Acetate + Carbon Dioxide + Water

STEP 1: Calculating Total Mass of Reactants
Mass of Reactants = Mass of Sodium Carbonate + Mass of Acetic Acid
Mass of Reactants = 5.3 g + 6.0 g = 11.3 g

STEP 2: Calculating Total Mass of Products
Mass of Products = Mass of Sodium Acetate + Mass of Carbon Dioxide + Mass of Water
Mass of Products = 8.2 g + 2.2 g + 0.9 g = 11.3 g

CONCLUSION:
Total Mass of Reactants (11.3 g) = Total Mass of Products (11.3 g).
Since there is zero change in mass during the chemical reaction, the data 
conclusively validates the Law of Conservation of Mass.
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Question 12 An element A (Atomic Number 11) combines with an element B (Atomic Number 17). What is the nature of the chemical bond and the formula of the compound? [Exam Favorite]

Answer:

• Element A (Z = 11, Sodium, Na): Electronic configuration = 2, 8, 1 (Metal, loses 1 electron to form A⁺).
• Element B (Z = 17, Chlorine, Cl): Electronic configuration = 2, 8, 7 (Non-metal, gains 1 electron to form B⁻).
• Nature of Bond: Ionic (Electrovalent) Bond, formed by the complete transfer of one valence electron from atom A to atom B.
• Chemical Formula: AB (e.g., NaCl, Sodium Chloride).

Question 13 Differentiate between an Atom and a Molecule, and between a Cation and an Anion with examples. [Exam Favorite]

Answer:

• Atom vs. Molecule:
  • Atom: The smallest unit of an element that retains its chemical identity and takes part in chemical reactions (e.g., He, Na, single O atom).
  • Molecule: An electrically neutral group of two or more atoms held together by covalent chemical bonds that can exist independently in a free state (e.g., O₂, H₂O, CO₂).
• Cation vs. Anion:
  • Cation: A positively charged ion formed when an atom loses one or more valence electrons (e.g., Na⁺, Ca²⁺, Al³⁺, NH₄⁺).
  • Anion: A negatively charged ion formed when an atom gains one or more valence electrons (e.g., Cl⁻, O²⁻, SO₄²⁻, NO₃⁻).

Question 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 molten state, while in 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. [Exam Favorite]

Answer: Correct Option: (iii) A is true, but R is false.

• Explanation:
  • Assertion (A) is TRUE: Copper sulfate (CuSO₄) is an ionic compound. In the solid state, its Cu²⁺ and SO₄²⁻ ions are locked rigidly in a crystal lattice and cannot move; hence, it is an electrical insulator. In the molten state (or aqueous solution), the lattice breaks down, freeing the ions to move and conduct current.
  • Reason (R) is FALSE: The reason states the exact opposite of reality. Ions are fixed in the solid state and free in the molten state.

Question 15 The species ²⁷Al, ⁸⁰Br⁻, and ²⁰¹Hg²⁺ have 13, 35, and 80 protons, respectively. Determine the number of protons, neutrons, and electrons in each. [Exam Favorite]

Answer:

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SUBATOMIC PARTICLE COUNT:
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1. Aluminium Atom (²⁷Al, Z = 13, A = 27, Neutral):
   • Protons   = 13
   • Electrons = 13 (Neutral atom)
   • Neutrons  = Mass Number (27) - Atomic Number (13) = 14 neutrons

2. Bromide Anion (⁸⁰Br⁻, Z = 35, A = 80, Charge = -1):
   • Protons   = 35
   • Electrons = 35 + 1 (Gained 1 electron) = 36 electrons
   • Neutrons  = Mass Number (80) - Atomic Number (35) = 45 neutrons

3. Mercury(II) Cation (²⁰¹Hg²⁺, Z = 80, A = 201, Charge = +2):
   • Protons   = 80
   • Electrons = 80 - 2 (Lost 2 electrons) = 78 electrons
   • Neutrons  = Mass Number (201) - Atomic Number (80) = 121 neutrons
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Frequently Asked Questions (FAQs) – Class 9 Science Chapter 9

Question 1: What is Atomicity? Give examples of monoatomic, diatomic, and polyatomic molecules. [Exam Favorite] Answer: Atomicity is the total number of atoms constituting a single molecule:

• Monoatomic (Atomicity = 1): Noble gases like Helium (He), Argon (Ar).
• Diatomic (Atomicity = 2): Oxygen (O₂), Hydrogen (H₂), Nitrogen (N₂).
• Triatomic (Atomicity = 3): Ozone (O₃), Carbon dioxide (CO₂).
• Polyatomic (Atomicity > 3): Phosphorus (P₄, tetra-atomic), Sulphur (S₈, octa-atomic).

Question 2: Define a Polyatomic Ion with two examples. [Exam Favorite] Answer: A polyatomic ion is a tightly bound group of atoms of different elements that carries a net positive or negative electrical charge and acts as a single chemical unit in reactions (e.g., Ammonium ion NH₄⁺, Carbonate ion CO₃²⁻, Sulphate ion SO₄²⁻).

Question 3: What is the molecular mass of Water (H₂O) and Carbon Dioxide (CO₂)? [Exam Favorite] Answer:

• Water (H₂O): (2 × 1 u) + (1 × 16 u) = 18 u.
• Carbon Dioxide (CO₂): (1 × 12 u) + (2 × 16 u) = 12 + 32 = 44 u.

Question 4: What is the ratio by mass of combining elements in Ammonia (NH₃)? [Exam Favorite] Answer: In Ammonia (NH₃), Nitrogen (14\text{ u}) and Hydrogen (3 \times 1 = 3\text{ u}) are always present in the fixed mass ratio of 14 : 3.

Question 5: Why do ionic compounds have high melting and boiling points? [Exam Favorite] Answer: Ionic compounds consist of closely packed cations and anions held together by extremely strong, multi-directional electrostatic forces of attraction. Breaking this rigid three-dimensional crystal lattice requires a large amount of heat energy.

Question 6: What is a Binary Compound? Give an example. [Exam Favorite] Answer: A binary compound is a simple chemical substance formed by the combination of only two different elements (e.g., Hydrogen chloride HCl, Magnesium oxide MgO, Hydrogen sulphide H₂S).

Question 7: How is the formula unit mass of Calcium Chloride (CaCl₂) calculated? [Exam Favorite] Answer: Mass = (1 × Atomic mass of Ca) + (2 × Atomic mass of Cl) = (1 × 40 u) + (2 × 35.5 u) = 40 + 71 = 111 u.

Question 8: What is an Electrovalent (Ionic) Bond? [Exam Favorite] Answer: An electrovalent bond is the chemical bond formed by the complete electrostatic transfer of one or more valence electrons from an electropositive metallic atom to an electronegative non-metallic atom.

Question 9: What is a Covalent Bond? [Exam Favorite] Answer: A covalent bond is a chemical bond formed by the mutual sharing of one or more electron pairs between two combining non-metal atoms to attain stable noble gas electron configurations.

Question 10: Name the elements present in: (a) Quick lime, (b) Baking soda. [Exam Favorite] Answer:

• (a) Quick Lime (CaO): Calcium and Oxygen.
• (b) Baking Soda (NaHCO₃): Sodium, Hydrogen, Carbon, and Oxygen.

Question 11: Why is the valency of the Noble gases zero? [Exam Favorite] Answer: Noble gases (Helium, Neon, Argon) have completely filled valence shells (Helium has 2; Neon/Argon have 8). They do not lose, gain, or share electrons, giving them a combining capacity of zero.

Question 12: What is the difference between 2N and N₂? [Exam Favorite] Answer:

• 2N: Represents two separate, uncombined nitrogen atoms.
• N₂: Represents one single molecule of nitrogen gas containing two nitrogen atoms chemically joined by a triple covalent bond.

Question 13: Write the chemical formula of: (a) Ammonium sulphate, (b) Lead nitrate. [Exam Favorite] Answer:

• (a) Ammonium sulphate: (NH₄)₂SO₄
• (b) Lead(II) nitrate: Pb(NO₃)₂

Question 14: Can a covalent compound like glucose conduct electricity in water? Why? [Exam Favorite] Answer: No. Glucose dissolves as neutral, non-ionized molecules (C₆H₁₂O₆). Because it produces no free mobile ions in water, the solution cannot carry an electric current.

Question 15: What is the mass ratio of Carbon to Oxygen in Carbon Monoxide (CO)? [Exam Favorite] Answer: In Carbon Monoxide (CO), Carbon (12\text{ u}) and Oxygen (16\text{ u}) are combined in the fixed mass ratio of 12 : 16 = 3 : 4.

Mastering the NCERT Solutions for Class 9 Science Chapter 9 (Exploration), “Atomic Foundations of Matter”, equips students with the chemical laws of combination, ion valency rules, formula derivation methods, and bonding principles required for top performance in CBSE chemistry evaluations. Review the numerical box solutions, the master summary tables, and the 15 high-yield FAQs above to secure full marks in your examinations.