Home · Schooling · CBSE · Class 9 · Science · Journey Inside the Atom · End-of-chapter exercises

NCERT Class 9 Science Journey Inside the Atom: Exercise Solutions

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

1. Exercises Q1

   About Rutherford's gold foil experiment, which are correct (explain each)? (i) it showed neutrons in the nucleus (ii) it disproved the plum pudding model and led to the idea of a central nucleus (iii) the large deflection of a few alpha particles showed most of the mass and positive charge packed in a tiny centre (iv) the deflections showed electrons move around the nucleus

   1. (i) Incorrect because neutrons were discovered later by Chadwick in 1932, not in this experiment.
   2. (ii) Correct because the unexpected scattering results could not be explained by Thomson's model.
   3. (iii) Correct because large deflections implied a concentrated positive charge and mass in a small region.
   4. (iv) Incorrect because the experiment focused on nuclear structure, not electron movement paths.

   Answer: (ii), (iii)

   Report an error in this solution

2. Exercises Q2

   By Bohr's model, which are correct (give a reason for each)? (i) electrons lose energy in fixed orbits and slowly fall into the nucleus (ii) electrons can be anywhere around the nucleus with no fixed energy (iii) electrons move in orbits of fixed energy without losing energy (iv) electrons can be found between energy levels

   1. (i) Incorrect, because Bohr stated that electrons do not lose energy while moving in a fixed shell.
   2. (ii) Incorrect, because electrons can only revolve in specific allowed shells with fixed energy levels.
   3. (iii) Correct, as the model says electrons move in orbits of fixed energy without losing energy.
   4. (iv) Incorrect, because electrons can revolve only in allowed shells and not in between them.

   Answer: (iii)

   Report an error in this solution

3. Exercises Q3

   Nuclei: X has 18 protons and 19 neutrons, Y 17 and 18, Z 17 and 20. How are (i) Y and Z related (ii) Z and X related?

   1. (i) Y and Z both have 17 protons, so they share the same atomic number but differ in neutron count, making them isotopes.
   2. (ii) Z has 17 protons and 20 neutrons for a mass number of 37, while X has 18 protons and 19 neutrons for a mass number of 37.
   3. (ii) Since X and Z have the same mass number but different atomic numbers, they are classified as isobars.

   Answer: (i) Y and Z are isotopes; (ii) Z and X are isobars

   Report an error in this solution

4. Exercises Q4

   From the few alpha particles that bounced back or turned through large angles, what did Rutherford conclude about where the positive part of the atom is and what it is like?

   1. The sharp deflection and bouncing back of a few alpha particles showed that they encountered a concentrated positive charge.
   2. Rutherford concluded that this positive part is located at the very centre of the atom in an extremely small region called the nucleus.
   3. He determined that this nucleus is dense and contains all the positive charge along with most of the atom's mass.

   Answer: The positive part is located at the centre in a tiny, dense nucleus that holds all positive charge and most of the mass.

   Report an error in this solution

5. Exercises Q5

   Arrange in historical order and explain: (i) Bohr — electrons in fixed orbits of definite energy (ii) Thomson — plum pudding (iii) Rutherford — dense central nucleus (iv) Dalton — indivisible atoms.

   1. (iv) Dalton's model described atoms as indivisible particles, which was the earliest model in the sequence.
   2. (ii) Thomson's model depicted the atom as a plum pudding with electrons embedded in a sphere of positive charge.
   3. (iii) Rutherford's model proposed that atoms have a dense central nucleus, improving upon Thomson's model.
   4. (i) Bohr's model proposed that electrons move in fixed orbits around the nucleus, each with a definite energy, addressing stability issues.

   Answer: (iv) Dalton; (ii) Thomson; (iii) Rutherford; (i) Bohr

   Report an error in this solution

6. Exercises Q6

   Why don't electrons moving around the nucleus fly away? What keeps them attracted to it?

   1. The nucleus of an atom contains protons, which carry a positive charge.
   2. Electrons are negatively charged particles that move around this positively charged nucleus.
   3. The opposite charges create an electrostatic force of attraction between the nucleus and the electrons.
   4. This attractive force holds the electrons in fixed circular paths or orbits around the nucleus.

   Answer: Electrons do not fly away because the negatively charged electrons are attracted to the positively charged nucleus by electrostatic force, keeping them in fixed orbits.

   Report an error in this solution

7. Exercises Q7

   Assertion: the discovery of subatomic particles helped explain atomic structure. Reason: an atom has as many electrons as protons. 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. (i) The assertion is correct because finding particles like protons and electrons revealed how atoms are built.
   2. (ii) The reason is correct because atoms are neutral, so the count of electrons matches the count of protons.
   3. (iii) The reason does not explain the assertion because charge balance is a property of the atom, not the cause of discovering its parts.
      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.

   Answer: (ii) Both statements are true, yet the reason does not explain the assertion.

   Report an error in this solution

8. Exercises Q8

   A magnesium atom has mass number 24 and atomic number 12. Find its protons, neutrons and electrons, and show its electron arrangement.

   1. The atomic number of magnesium is 12, so it has 12 protons.
   2. Since the atom is neutral, the number of electrons equals the number of protons, which is 12.
   3. The mass number is 24, so the number of neutrons is 24 − 12 = 12.
   4. The first shell (K) holds 2 electrons, the second shell (L) holds 8 electrons, and the remaining 2 electrons go in the third shell (M).
   5. The electron arrangement is 2, 8, 2.

   Answer: Protons: 12; Neutrons: 12; Electrons: 12; Electron arrangement: 2, 8, 2

   Report an error in this solution

9. Exercises Q9

   Fig. 8.17 shows four atoms with electrons in shells (a) 2, 1 (b) 2, 5 (c) 2, 8, 3 (d) 2, 7. For each give the name, symbol, total electrons, valence electrons, valency, protons and atomic number.

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

   1. (a) The electron configuration is 2, 1. The total number of electrons is 2 + 1 = 3. The atomic number is 3, which corresponds to Lithium (Li). The valence electrons are 1, so the valency is 1. The number of protons is 3.
   2. (b) The electron configuration is 2, 5. The total number of electrons is 2 + 5 = 7. The atomic number is 7, which corresponds to Nitrogen (N). The valence electrons are 5, so the valency is 3 (8 - 5). The number of protons is 7.
   3. (c) The electron configuration is 2, 8, 3. The total number of electrons is 2 + 8 + 3 = 13. The atomic number is 13, which corresponds to Aluminium (Al). The valence electrons are 3, so the valency is 3. The number of protons is 13.
   4. (d) The electron configuration is 2, 7. The total number of electrons is 2 + 7 = 9. The atomic number is 9, which corresponds to Fluorine (F). The valence electrons are 7, so the valency is 1 (8 - 7). The number of protons is 9.

   Answer: (a) Lithium (Li), 3 electrons, 1 valence electron, valency 1, 3 protons, atomic number 3; (b) Nitrogen (N), 7 electrons, 5 valence electrons, valency 3, 7 protons, atomic number 7; (c) Aluminium (Al), 13 electrons, 3 valence electrons, valency 3, 13 protons, atomic number 13; (d) Fluorine (F), 9 electrons, 7 valence electrons, valency 1, 9 protons, atomic number 9

   Report an error in this solution

10. Exercises Q10

    Both Rutherford's and Bohr's models have electrons orbiting the nucleus. Why could Rutherford's not explain the atom's stability while Bohr's could?

    1. Rutherford's model showed electrons moving in circular paths around the nucleus.
    2. A charged particle moving in a circle is constantly accelerating and changing direction.
    3. According to physics, an accelerating charged particle should lose energy continuously.
    4. Losing energy would cause the electron to spiral inward and fall into the nucleus.
    5. This collapse would make atoms unstable, which contradicts the fact that matter is stable.
    6. Bohr's model proposed that electrons move in specific, fixed orbits without radiating energy.
    7. In these fixed orbits, electrons do not lose energy, so they do not spiral into the nucleus.
    8. Thus, Bohr's model explains why atoms remain stable while Rutherford's does not.

    Answer: Rutherford's model failed because accelerating electrons should lose energy and collapse into the nucleus, while Bohr's model succeeded by proposing fixed orbits where electrons do not lose energy.

    Report an error in this solution

11. Exercises Q11

    An atom X with mass number 70 has 31 electrons. How many neutrons does it have?

    1. The number of electrons in a neutral atom is equal to the number of protons.
    2. Since atom X has 31 electrons, it has 31 protons.
    3. The atomic number (Z) is therefore 31.
    4. The mass number (A) is given as 70.
    5. The number of neutrons is calculated by subtracting the atomic number from the mass number.
    6. Number of neutrons = Mass number − Atomic number
    7. Number of neutrons = 70 − 31
    8. Number of neutrons = 39

    Answer: 39

    Report an error in this solution

12. Exercises Q12

    An atom has 79 protons and mass number 197. Find (i) its neutrons (ii) its electrons.

    1. (i) The mass number is the sum of protons and neutrons, so neutrons = mass number − protons.
    2. (i) So neutrons = 197 − 79.
    3. (i) That is 118 neutrons.
    4. (ii) In a neutral atom, the number of electrons equals the number of protons.
    5. (ii) Since the atom has 79 protons, it has 79 electrons.

    Answer: (i) 118; (ii) 79

    Report an error in this solution

13. Exercises Q13

    Complete the table (atomic number, mass number, neutrons, protons, electrons, element): row 1: Z = 5, 6 neutrons; row 2: mass number 14, 7 electrons, nitrogen; row 3: mass number 24, 12 protons; row 4: Z = 15, 16 neutrons; row 5: mass number 1, 0 neutrons.

    1. For row 1, atomic number Z is 5, so protons are 5 and electrons are 5.
    2. The element with Z = 5 is Boron.
    3. Mass number is protons plus neutrons, so 5 + 6 = 11.
    4. For row 2, electrons are 7, so protons are 7 and atomic number Z is 7.
    5. The element with Z = 7 is Nitrogen.
    6. Neutrons are mass number minus protons, so 14 - 7 = 7.
    7. For row 3, protons are 12, so atomic number Z is 12 and electrons are 12.
    8. The element with Z = 12 is Magnesium.
    9. Neutrons are mass number minus protons, so 24 - 12 = 12.
    10. For row 4, atomic number Z is 15, so protons are 15 and electrons are 15.
    11. The element with Z = 15 is Phosphorus.
    12. Mass number is protons plus neutrons, so 15 + 16 = 31.
    13. For row 5, mass number is 1 and neutrons are 0, so protons are 1 - 0 = 1.
    14. Atomic number Z is 1, so electrons are 1.
    15. The element with Z = 1 is Hydrogen.

    Answer: (i) 11, 5, 5, Boron; (ii) 7, 7, 7, Nitrogen; (iii) 12, 12, 12, Magnesium; (iv) 31, 15, 15, Phosphorus; (v) 1, 1, 1, Hydrogen

    Report an error in this solution

14. Exercises Q14

    Element X has mass number 35 and 18 neutrons. (i) Its electrons and protons? (ii) Atomic number? (iii) Which element? (iv) Electronic configuration? (v) Valence electrons? (vi) Mass number if two neutrons are added? (vii) How is X related to the new atom?

    1. (i) Mass number = protons + neutrons, so protons = 35 − 18 = 17.
    2. (i) In a neutral atom, electrons equal protons, so electrons = 17.
    3. (ii) The atomic number is the number of protons, which is 17.
    4. (iii) The element with atomic number 17 is Chlorine.
    5. (iv) The electronic configuration of Chlorine (17 electrons) is 2, 8, 7.
    6. (v) The valence electrons are the electrons in the outermost shell, which is 7.
    7. (vi) If two neutrons are added, the new mass number is 35 + 2 = 37.
    8. (vii) The new atom has the same protons (17) but different neutrons, so they are isotopes.

    Answer: (i) 17 electrons and 17 protons; (ii) 17; (iii) Chlorine; (iv) 2, 8, 7; (v) 7; (vi) 37; (vii) isotopes

    Report an error in this solution

15. Exercises Q15

    An atom has 12 protons and 12 neutrons. If its electrons were replaced by particles with the same charge but 500 times heavier, what happens to its (i) atomic number (ii) atomic mass (iii) mass number (iv) overall charge?

    1. (i) The atomic number depends only on the number of protons, which remains 12, so it does not change.
    2. (ii) The atomic mass increases because the new particles are 500 times heavier than the original electrons.
    3. (iii) The mass number is the sum of protons and neutrons, so it remains 24 as the nucleus is unchanged.
    4. (iv) The overall charge remains zero because the number of negative particles equals the number of protons.

    Answer: (i) No change; (ii) Increases; (iii) No change; (iv) No change (remains neutral)

    Report an error in this solution

← Journey Inside the Atom: notes, practice and all exercises