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NCERT Class 9 Science Atomic Foundations of Matter: Exercise Solutions

Step-by-step working for every question of End-of-chapter exercises in NCERT's Class 9 Science book, chapter “Atomic Foundations of Matter”, written by Shishya. Each question is in our own short words; open the NCERT chapter for the exact wording and figures.

1. Exercises Q1

   Element A has one electron in its third shell; element B has six electrons in its second shell. (i) How many electrons does A give or take to become stable? (ii) What ion does it form? (iii) and (iv) The same for B. (v) What bond forms when A and B combine? (vi) The formula of the compound?

   1. Element A has one electron in its third shell, so its electronic configuration is 2, 8, 1.
   2. To become stable, A loses its single outermost electron.
   3. After losing one electron, A forms a cation with a charge of +1, written as A⁺.
   4. Element B has six electrons in its second shell, so its electronic configuration is 2, 6.
   5. To become stable, B gains two electrons to complete its octet.
   6. After gaining two electrons, B forms an anion with a charge of −2, written as B²⁻.
   7. Since A is a metal and B is a non-metal, they combine by transferring electrons to form an ionic bond.
   8. The charges +1 and −2 balance in a ratio of two A atoms to one B atom.
      why?

      A balanced equation has the same number of atoms of each element on both sides, because atoms are neither created nor destroyed in a chemical reaction (law of conservation of mass). Only the numbers in front (coefficients) change, never the formulas.

   9. The chemical formula of the compound is A₂B.

   Answer: (i) A gives 1 electron; (ii) A⁺; (iii) B takes 2 electrons; (iv) B²⁻; (v) ionic; (vi) A₂B

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2. Exercises Q2

   Element X has six outer electrons and forms a diatomic molecule. (i) Why? (ii) What kind of bond? (iii) Draw the molecule. (iv) Element Y has two electrons in its second shell: draw the molecule X forms with Y.

   1. (i) Element X has six valence electrons and needs two more to complete its octet, so two atoms share electrons to become stable.
      why?

      In a covalent bond two atoms share a pair of electrons so that each gets a full outer shell. Carbon forms such bonds because gaining or losing four electrons would need too much energy.

   2. (ii) The atoms share two pairs of electrons, forming a double covalent bond.
      why?

      In a covalent bond two atoms share a pair of electrons so that each gets a full outer shell. Carbon forms such bonds because gaining or losing four electrons would need too much energy.

   3. (iii) The structure is represented as X=X, showing two lines between the atoms.
   4. (iv) Y has the arrangement 2, 2, so it has two valence electrons, which it gives away; X takes these two electrons to complete its octet. Y becomes Y²⁺ and X becomes X²⁻, so they form an ionic compound YX: draw Y with its two outer electrons moving to X, giving [Y]²⁺ [X]²⁻.

   Answer: (i) Each X atom needs two more electrons, so two atoms share electrons to complete their octets; (ii) a double covalent bond; (iii) X=X; (iv) Y gives its two valence electrons to X, forming the ionic compound YX (Y²⁺ X²⁻).

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3. Exercises Q3

   For an ionic compound with total charges 6+ and 6−, which set of ions is right? (i) 2 Al³⁺ and 3 Cl⁻ (ii) 3 Mg²⁺ and 1 PO₄³⁻ (iii) 2 Fe³⁺ and 3 O²⁻ (iv) 3 Ca²⁺ and 2 SO₄²⁻

   1. Check option (i): 2 Al³⁺ gives 6+ charge, but 3 Cl⁻ gives only 3- charge, so the charges do not balance.
   2. Check option (ii): 3 Mg²⁺ gives 6+ charge, but 1 PO₄³⁻ gives only 3- charge, so the charges do not balance.
   3. Check option (iii): 2 Fe³⁺ gives 6+ charge and 3 O²⁻ gives 6- charge, so the total positive and negative charges are equal.
   4. Check option (iv): 3 Ca²⁺ gives 6+ charge and 2 SO₄²⁻ gives 4- charge, so the charges do not balance.

   Answer: (iii)

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4. Exercises Q4

   Pick the correct statements and correct the false ones: (i) elements are made of molecules and compounds of atoms (ii) a compound's molecule always has two or more atoms of the same kind (iii) a nitrogen gas molecule has three nitrogen atoms (iv) water is two hydrogen atoms covalently bonded to one oxygen atom.

   1. (i) False: an element is made of only one kind of atom (as single atoms or as molecules like O₂), and a compound is made of atoms of two or more different elements joined together.
   2. (ii) False: a molecule of a compound always has atoms of two or more different kinds, such as H₂O.
   3. (iii) The statement is false because a nitrogen gas molecule consists of two nitrogen atoms, not three.
   4. (iv) The statement is correct as water molecules are formed by two hydrogen atoms covalently bonded to one oxygen atom.
      why?

      In a covalent bond two atoms share a pair of electrons so that each gets a full outer shell. Carbon forms such bonds because gaining or losing four electrons would need too much energy.

   Answer: (iv)

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5. Exercises Q5

   Write the formulae of (i) aluminium nitrate (ii) calcium oxide (iii) ferric oxide.

   1. (i) Aluminium has a valency of +3 and the nitrate ion (NO₃) has a charge of -1.
   2. (i) To balance the charges, three nitrate ions are needed for one aluminium ion.
   3. (i) The formula is Al(NO₃)₃.
   4. (ii) Calcium has a valency of +2 and oxygen has a valency of -2.
   5. (ii) The charges balance in a 1:1 ratio.
   6. (ii) The formula is CaO.
   7. (iii) Ferric refers to iron with a valency of +3 and oxygen has a valency of -2.
   8. (iii) To balance the charges, two iron ions (+3 each) are needed for three oxygen ions (-2 each).
   9. (iii) The formula is Fe₂O₃.

   Answer: (i) Al(NO3)3; (ii) CaO; (iii) Fe2O3

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6. Exercises Q6

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

   1. (i) The charges are Ca²⁺ and Br⁻. To balance the +2 and -1 charges, we need two bromide ions for every calcium ion. The formula is CaBr2.
   2. (ii) The charges are Al³⁺ and CO₃²⁻. The least common multiple of 3 and 2 is 6. We need two aluminium ions (total +6) and three carbonate ions (total -6). The formula is Al2(CO3)3.
   3. (iii) The charges are K⁺ and SO₄²⁻. To balance the +1 and -2 charges, we need two potassium ions for every sulfate ion. The formula is K2SO4.
   4. (iv) The charges are NH₄⁺ and Cl⁻. The +1 and -1 charges balance directly in a 1:1 ratio. The formula is NH4Cl.

   Answer: (i) CaBr2; (ii) Al2(CO3)3; (iii) K2SO4; (iv) NH4Cl

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7. Exercises Q7

   Which of the four drawings in Fig. 9.18 shows a Cl⁻ ion correctly (chlorine's atomic number is 17)? (i) (ii) (iii) (iv)

   Uses a figure printed in the NCERT book — open the chapter above to see it.

   1. (i) A chlorine atom has an atomic number of 17, meaning it contains 17 protons and 17 electrons in its neutral state.
   2. (ii) To form a chloride ion (Cl⁻), the atom gains one extra electron to complete its octet, resulting in a total of 18 electrons.
   3. (iii) The correct drawing must show 17 protons in the nucleus and 18 electrons in the shells, with a negative charge indicated.
   4. (iv) Drawing (ii) is the only option that displays this specific arrangement of 17 protons and 18 electrons.

   Answer: (ii)

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8. Exercises Q8

   Find the formula unit mass of (i) ammonium nitrate NH₄NO₃ (ii) phosphoric acid H₃PO₄ (iii) sodium hydrogencarbonate NaHCO₃ (H = 1 u, C = 12 u, N = 14 u, O = 16 u, Na = 23 u, P = 31 u).

   1. (i) The formula of ammonium nitrate is NH₄NO₃, which contains 2 nitrogen atoms, 4 hydrogen atoms, and 3 oxygen atoms.
   2. (i) Calculate the mass: (2 × 14) + (4 × 1) + (3 × 16) = 28 + 4 + 48 = 80 u.
   3. (ii) The formula of phosphoric acid is H₃PO₄, which contains 3 hydrogen atoms, 1 phosphorus atom, and 4 oxygen atoms.
   4. (ii) Calculate the mass: (3 × 1) + (1 × 31) + (4 × 16) = 3 + 31 + 64 = 98 u.
   5. (iii) The formula of sodium hydrogencarbonate is NaHCO₃, which contains 1 sodium atom, 1 hydrogen atom, 1 carbon atom, and 3 oxygen atoms.
   6. (iii) Calculate the mass: (1 × 23) + (1 × 1) + (1 × 12) + (3 × 16) = 23 + 1 + 12 + 48 = 84 u.

   Answer: (i) 80 u; (ii) 98 u; (iii) 84 u

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9. Exercises Q9

   Write the formulae of the compounds formed by (i) magnesium and nitrogen (ii) lithium and nitrogen (iii) sodium and sulfur (iv) aluminium and oxygen.

   1. (i) Magnesium has a valency of 2 and nitrogen has a valency of 3, so the formula is Mg3N2.
   2. (ii) Lithium has a valency of 1 and nitrogen has a valency of 3, so the formula is Li3N.
   3. (iii) Sodium has a valency of 1 and sulfur has a valency of 2, so the formula is Na2S.
   4. (iv) Aluminium has a valency of 3 and oxygen has a valency of 2, so the formula is Al2O3.

   Answer: (i) Mg3N2; (ii) Li3N; (iii) Na2S; (iv) Al2O3

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10. Exercises Q10

    Write the formula of each compound of the cations NH₄⁺, Li⁺, Al³⁺, Cu²⁺ with the anions NO₃⁻, SO₄²⁻, PO₄³⁻ (LiNO₃ is the example).

    1. The formula of a compound is written by balancing the total positive charge of cations with the total negative charge of anions.
    2. For NH4+ and NO3-, both have a charge of 1, so they combine in a 1:1 ratio to form NH4NO3.
    3. For NH4+ and SO42-, two NH4+ ions (total +2) are needed to balance one SO42- ion (total -2), forming (NH4)2SO4.
    4. For NH4+ and PO43-, three NH4+ ions (total +3) are needed to balance one PO43- ion (total -3), forming (NH4)3PO4.
    5. For Li+ and NO3-, both have a charge of 1, so they combine in a 1:1 ratio to form LiNO3.
    6. For Li+ and SO42-, two Li+ ions (total +2) are needed to balance one SO42- ion (total -2), forming Li2SO4.
    7. For Li+ and PO43-, three Li+ ions (total +3) are needed to balance one PO43- ion (total -3), forming Li3PO4.
    8. For Al3+ and NO3-, one Al3+ ion (total +3) is balanced by three NO3- ions (total -3), forming Al(NO3)3.
    9. For Al3+ and SO42-, two Al3+ ions (total +6) are balanced by three SO42- ions (total -6), forming Al2(SO4)3.
    10. For Al3+ and PO43-, both have a charge magnitude of 3, so they combine in a 1:1 ratio to form AlPO4.
    11. For Cu2+ and NO3-, one Cu2+ ion (total +2) is balanced by two NO3- ions (total -2), forming Cu(NO3)2.
    12. For Cu2+ and SO42-, both have a charge magnitude of 2, so they combine in a 1:1 ratio to form CuSO4.
    13. For Cu2+ and PO43-, three Cu2+ ions (total +6) are balanced by two PO43- ions (total -6), forming Cu3(PO4)2.

    Answer: NH4NO3, (NH4)2SO4, (NH4)3PO4, LiNO3, Li2SO4, Li3PO4, Al(NO3)3, Al2(SO4)3, AlPO4, Cu(NO3)2, CuSO4, Cu3(PO4)2

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11. Exercises Q11

    5.3 g sodium carbonate and 6.0 g acetic acid give 2.2 g carbon dioxide, 0.9 g water and 8.2 g sodium acetate. Does this obey the law of conservation of mass?

    1. Calculate the total mass of the reactants by adding the mass of sodium carbonate and acetic acid.
    2. Mass of reactants = 5.3 g + 6.0 g = 11.3 g.
    3. Calculate the total mass of the products by adding the masses of carbon dioxide, water, and sodium acetate.
    4. Mass of products = 2.2 g + 0.9 g + 8.2 g = 11.3 g.
    5. Compare the total mass of reactants with the total mass of products.
    6. Since 11.3 g is equal to 11.3 g, the mass is conserved.
    7. Therefore, the reaction obeys the law of conservation of mass.

    Answer: Yes, it obeys the law of conservation of mass as total mass of reactants (11.3 g) equals total mass of products (11.3 g).

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12. Exercises Q12

    A species has 11 protons, 12 neutrons and 10 electrons. (i) Its atomic number and mass number? (ii) Neutral, cation or anion? Why? (iii) Electronic configuration? (iv) Name it.

    1. (i) The atomic number is the number of protons, which is 11.
    2. (i) The mass number is the sum of protons and neutrons, so 11 + 12 = 23.
    3. (ii) The species has 11 protons and 10 electrons.
    4. (ii) Since there are more protons than electrons, it carries a positive charge and is a cation.
    5. (iii) The electronic configuration is determined by the number of electrons, which is 10.
    6. (iii) The electrons fill the shells as 2 in the first shell and 8 in the second shell, giving 2, 8.
    7. (iv) An element with atomic number 11 is Sodium.
    8. (iv) The species is a sodium ion.

    Answer: (i) Atomic number 11, mass number 23; (ii) cation; (iii) 2, 8; (iv) Sodium

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13. Exercises Q13

    Element A is 2, 8, 5 and element B is 2, 8, 7. (i) Which is more reactive? (ii) Will they form an ionic or a covalent bond? Explain by transfer or sharing of electrons. (iii) Predict the formula of their compound.

    1. (i) Element B is more reactive because it has seven valence electrons and needs only one more to become stable, whereas element A needs three.
    2. (ii) They form a covalent bond because both are non-metals that share electrons to complete their valence shells rather than transferring them.
       why?

       In a covalent bond two atoms share a pair of electrons so that each gets a full outer shell. Carbon forms such bonds because gaining or losing four electrons would need too much energy.

    3. (iii) Element A shares three electrons with three atoms of element B, each sharing one electron, resulting in the formula AB3.

    Answer: (i) Element B; (ii) covalent; (iii) AB3

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14. Exercises Q14

    Assertion: copper sulfate conducts electricity when molten but not when solid. Reason: its ions are fixed in the lattice when molten and free to move when solid. Options: (i) both true, R explains A (ii) both true, R does not explain A (iii) A true, R false (iv) A false, R true

    1. The assertion is true because copper sulfate conducts electricity when molten but not when solid.
    2. The reason is false because ions are fixed in the solid state, not the molten state.
    3. In the solid state, ions are held in fixed positions by strong forces and cannot move to carry charge.
    4. In the molten state, ions are free to move, which allows the substance to conduct electricity.

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

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15. Exercises Q15

    ²⁷Al, ⁸⁰Br⁻ and ²⁰¹Hg²⁺ have 13, 35 and 80 protons. How many electrons and neutrons does each have?

    1. For ²⁷Al, the atomic number is 13, so it has 13 protons and 13 electrons in a neutral atom.
    2. The number of neutrons in ²⁷Al is mass number minus atomic number, which is 27 − 13 = 14.
    3. For ⁸⁰Br⁻, the atomic number is 35, so it has 35 protons.
    4. Since ⁸⁰Br⁻ is a negative ion with a charge of −1, it has gained one electron, so it has 35 + 1 = 36 electrons.
    5. The number of neutrons in ⁸⁰Br⁻ is 80 − 35 = 45.
    6. For ²⁰¹Hg²⁺, the atomic number is 80, so it has 80 protons.
    7. Since ²⁰¹Hg²⁺ is a positive ion with a charge of +2, it has lost two electrons, so it has 80 − 2 = 78 electrons.
    8. The number of neutrons in ²⁰¹Hg²⁺ is 201 − 80 = 121.

    Answer: ²⁷Al has 13 electrons and 14 neutrons; ⁸⁰Br⁻ has 36 electrons and 45 neutrons; ²⁰¹Hg²⁺ has 78 electrons and 121 neutrons.

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