Question Bank — Journey Inside the Atom

NCERT Class 9 Science · Chapter 8 · CBSE Board Pattern

Class 9 Science Chapter 8 Question Bank CBSE Board Pattern

Section A — MCQs (10 questions, 1 mark each)

  1. Who proposed the idea of ‘parmanu’ as the smallest indivisible particle of matter?
    (a) Democritus (b) Leucippus (c) Acharya Kanada (d) John Dalton

  2. In Thomson’s model of the atom, electrons are:
    (a) revolving around the nucleus (b) embedded in a sphere of positive charge (c) present only in the nucleus (d) absent from the atom

  3. The gold foil experiment was performed by:
    (a) J.J. Thomson (b) Niels Bohr (c) Geiger and Marsden under Rutherford (d) James Chadwick

  4. Rutherford concluded that the nucleus of an atom is:
    (a) negatively charged and large (b) positively charged, dense and very small (c) empty space (d) made only of electrons

  5. According to Bohr’s model, electrons revolve in:
    (a) random paths (b) fixed circular paths called shells or energy levels (c) straight lines (d) elliptical paths only

  6. The subatomic particle discovered by James Chadwick is:
    (a) electron (b) proton (c) neutron (d) positron

  7. Assertion (A): Rutherford’s model could not explain the stability of atoms.
    Reason (R): According to Rutherford, electrons moving in circular paths around the nucleus should continuously lose energy and spiral into the nucleus.
    (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.

  8. Assertion (A): Atoms of different elements are distinguished by their atomic number.
    Reason (R): Atomic number is the number of protons present in the nucleus of an atom.
    (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.

  9. The maximum number of electrons that can be accommodated in the L-shell (n = 2) is:
    (a) 2 (b) 8 (c) 18 (d) 32

  10. Atoms having the same atomic number but different mass numbers are called:
    (a) isobars (b) isotopes (c) isomers (d) isotones

Section B — Very Short Answer (6 questions, 2 marks each)

  1. Define the terms ‘parmanu’ and ‘atomos’. How are they related to the modern concept of an atom?
  2. State two main postulates of Dalton’s atomic theory.
  3. What are cathode rays? Name the subatomic particle discovered through their study.
  4. Write the electronic configuration of elements with atomic numbers 11 and 17.
  5. Differentiate between atomic number and mass number.
  6. What is valency? How is it related to the number of valence electrons?

Section C — Short Answer (5 questions, 3 marks each)

  1. Describe Thomson’s plum-pudding (or watermelon) model of the atom. Why was it proposed?
  2. State three main conclusions drawn by Rutherford from the gold-foil experiment.
  3. Explain why Rutherford’s model could not account for the stability of atoms.
  4. Write the rules proposed by Bohr and Bury for the distribution of electrons in different shells.
  5. What are isotopes? Give two examples and state why isotopes have similar chemical properties.

Section D — Long Answer (3 questions, 5 marks each)

  1. Describe the gold-foil experiment performed by Geiger and Marsden. Draw a labelled diagram showing the observations and state the conclusions that led to Rutherford’s nuclear model of the atom.
  2. (a) Define atomic number and mass number.
    (b) An atom has 17 protons and 18 neutrons. Write its symbol in the standard notation and find its electronic configuration and valency.
    (c) Calculate the number of atoms that would be stacked to form a sheet of paper 0.1 mm thick if the diameter of one atom is 10⁻¹⁰ m. (Show full working.)
  3. (a) What are isotopes? Explain with the help of the isotopes of hydrogen.
    (b) Chlorine occurs in nature as two isotopes: ³⁵Cl (75 %) and ³⁷Cl (25 %). Calculate the average atomic mass of chlorine. (Show full working.)
    (c) How are isobars different from isotopes? Give one example of isobars.

Section E — Case/Source-Based (2 questions, 4 marks each)

Case 1

Read the passage and answer the questions that follow:
“Rutherford’s gold-foil experiment showed that most α-particles passed straight through the foil while a few were deflected at large angles and some even bounced back. This led to the idea that the positive charge and most of the mass of the atom are concentrated in a very small region called the nucleus.”

(i) What does the undeflected passage of most α-particles indicate about the structure of the atom? (1)
(ii) Why did a few α-particles bounce back? (1)
(iii) How does this observation contradict Thomson’s model? (1)
(iv) State the approximate ratio of the diameter of the nucleus to that of the atom as given by Rutherford. (1)

Case 2

Read the passage and answer the questions that follow:
“Two isotopes of uranium, ²³⁵U and ²³⁸U, have the same number of protons but different numbers of neutrons. ²³⁵U is used as fuel in nuclear reactors. Similarly, ¹³¹I is used in the treatment of thyroid disorders.”

(i) What is the difference in the number of neutrons between ²³⁵U and ²³⁸U? (1)
(ii) Why do isotopes of an element show identical chemical properties? (1)
(iii) Name the scientist who discovered the neutron and state its relative charge. (1)
(iv) Give one more application of an isotope mentioned in the chapter. (1)

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Section A

  1. (c)
  2. (b)
  3. (c)
  4. (b)
  5. (b)
  6. (c)
  7. (i) — Both true, R explains A (definition of limitation — ½ mark, reason of energy loss — ½ mark)
  8. (i) — Both true, R explains A (definition of atomic number — ½ mark, role in identifying elements — ½ mark)
  9. (b)
  10. (b)

Section B

  1. Parmanu (Acharya Kanada) — smallest indivisible particle (1 mark); atomos (Democritus) — indivisible (1 mark).
  2. Matter made of indivisible atoms; atoms of same element identical (any two postulates — 1 mark each).
  3. Rays from cathode to anode in low-pressure gas tube (1 mark); electron (1 mark).
  4. Na (2,8,1); Cl (2,8,7) — 1 mark each.
  5. Atomic number = no. of protons (1 mark); mass number = protons + neutrons (1 mark).
  6. Combining capacity of an atom (1 mark); equal to electrons gained/lost/shared to complete octet (1 mark).

Section C

  1. Positive sphere with electrons embedded (plum-pudding/watermelon) (1 mark); to explain neutrality after discovering electrons (1 mark); balanced +ve and –ve charges (1 mark).
  2. Atom mostly empty space; nucleus dense, +vely charged and contains most mass; electrons revolve around nucleus (1 mark each).
  3. Electrons in circular paths accelerate and lose energy, should spiral into nucleus (2 marks); atoms would collapse, contradicting observed stability (1 mark).
  4. Max electrons in shell = 2n²; outermost shell max 8 electrons; filled stepwise from K shell outward (1 mark each).
  5. Same atomic number, different mass number (1 mark); ¹H, ²H, ³H or ¹²C, ¹³C, ¹⁴C (1 mark); same no. of electrons/valence electrons (1 mark).

Section D

  1. Description of setup and observations (2 marks); labelled diagram (beam, gold foil, detector, deflections) (2 marks); conclusions (nucleus, empty space) (1 mark).
  2. (a) Definitions (1 mark); (b) ¹⁷₁₇Cl, config 2,8,7, valency 1 (2 marks); (c) 10⁻⁴ / 10⁻¹⁰ = 10⁶ atoms (2 marks).
  3. (a) Definition + H isotopes (2 marks); (b) 35×0.75 + 37×0.25 = 35.5 u (2 marks); (c) Same mass number, different atomic number; e.g., ⁴⁰₁₈Ar & ⁴⁰₂₀Ca (1 mark).

Section E

Case 1

(i) Most of atom is empty space (1 mark).
(ii) Hit dense positive nucleus (1 mark).
(iii) Positive charge not spread uniformly (1 mark).
(iv) Nucleus ~10⁵ times smaller (1 mark).

Case 2

(i) 3 neutrons (1 mark).
(ii) Same number of electrons/electronic configuration (1 mark).
(iii) James Chadwick; charge 0 (1 mark).
(iv) ⁶⁰Co (cancer therapy) or ¹⁴C (dating) (1 mark).

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All questions are answerable from the NCERT chapter text. Reviewed by GFIS faculty.