Structure of the Atom Part 3: MCQs (Q51–Q75)
🧪 Structure of the Atom – Class 9 Science MCQs
Section 3: Atomic Number, Mass Number, Isotopes & Isobars (Q51–Q75)
Q51. Atomic number of an element represents:
A. Number of neutrons
B. Number of protons
C. Number of nucleons
D. Mass of atom
✅ Correct Answer: B. Number of protons
Explanation:
- A. Neutrons → Wrong, not atomic number.
- B. Correct, atomic number (Z) = protons = electrons (neutral atom).
- **C. Nucleons = protons + neutrons → mass number.
- **D. Wrong.
Q52. Mass number of an atom is equal to:
A. Protons only
B. Protons + Neutrons
C. Electrons + Protons
D. Electrons only
✅ Correct Answer: B. Protons + Neutrons
Explanation:
- A. Protons only → Wrong.
- B. Correct, A = p + n (nucleons).
- **C. Wrong, electrons not included.
- **D. Wrong.
Q53. For a neutral atom, number of electrons = ?
A. Neutrons
B. Protons
C. Mass number
D. Nucleons
✅ Correct Answer: B. Protons
Explanation:
- A. Neutrons → Not always equal.
- B. Correct, neutrality means protons = electrons.
- **C. Mass number → p+n, not equal to electrons.
- **D. Wrong.
Q54. Which of the following is NOT true?
A. Isotopes have same Z, different A
B. Isobars have same A, different Z
C. Atomic number = protons
D. Neutrons = protons in all atoms
✅ Correct Answer: D. Neutrons = protons in all atoms
Explanation:
- A. True for isotopes.
- B. True for isobars.
- C. Correct definition.
- **D. Wrong, neutrons vary.
Q55. Example of isotopes is:
A. ¹H, ²H, ³H
B. ¹²C, ¹⁴N
C. ²⁴Mg, ²⁴Na
D. ⁴⁰Ca, ⁴⁰Ar
✅ Correct Answer: A. ¹H, ²H, ³H
Explanation:
- A. Correct, same Z (1), different A.
- **B. Wrong, C and N are different elements.
- **C. Wrong, different Z.
- **D. Wrong, Ca and Ar are isobars.
Q56. Example of isobars is:
A. ¹²C, ¹³C
B. ²⁰Ne, ²¹Ne
C. ⁴⁰Ca, ⁴⁰Ar
D. ¹H, ²H
✅ Correct Answer: C. ⁴⁰Ca, ⁴⁰Ar
Explanation:
- A. Isotopes.
- **B. Isotopes of Ne.
- C. Correct, same A=40, different Z.
- **D. Isotopes of H.
Q57. Which of the following is used as a nuclear fuel isotope?
A. U-235
B. C-14
C. I-131
D. Co-60
✅ Correct Answer: A. U-235
Explanation:
- A. Correct, U-235 is nuclear fuel.
- **B. C-14 used in carbon dating.
- **C. I-131 used in thyroid treatment.
- **D. Co-60 used in radiotherapy.
Q58. Which isotope is used in carbon dating?
A. C-12
B. C-13
C. C-14
D. C-16
✅ Correct Answer: C. C-14
Explanation:
- **A. C-12 stable.
- **B. C-13 stable.
- **C. Correct, radioactive isotope used for dating fossils.
- **D. Not an isotope of carbon.
Q59. Which isotope is used in treatment of thyroid disorders?
A. I-131
B. U-235
C. C-14
D. Co-60
✅ Correct Answer: A. I-131
Explanation:
- A. Correct, iodine-131 used for thyroid treatment.
- **B. Nuclear fuel.
- **C. Carbon dating.
- **D. Cancer treatment.
Q60. Which isotope is used in radiotherapy for cancer?
A. U-235
B. I-131
C. Co-60
D. C-14
✅ Correct Answer: C. Co-60
Explanation:
- **A. Wrong, nuclear fuel.
- **B. Wrong, thyroid.
- **C. Correct, cobalt-60 used for cancer radiotherapy.
- **D. Wrong, carbon dating.
Q61. Atomic number of sodium is 11. Its electronic configuration is:
A. 2,8,1
B. 2,7,2
C. 2,6,3
D. 2,8,2
✅ Correct Answer: A. 2,8,1
Explanation:
- Sodium (Z=11) → electrons = 11 → fill shells = 2,8,1.
Q62. Number of neutrons in ³⁷Cl (Z=17):
A. 17
B. 20
C. 37
D. 54
✅ Correct Answer: B. 20
Explanation:
- Neutrons = A – Z = 37 – 17 = 20.
Q63. Number of protons in ³⁷Cl:
A. 17
B. 20
C. 37
D. 54
✅ Correct Answer: A. 17
Explanation:
- Protons = Z = 17.
Q64. Which of the following are isotopes of hydrogen?
A. Protium, Deuterium, Tritium
B. H, He, Li
C. ²H, ³He
D. C-12, C-14
✅ Correct Answer: A. Protium, Deuterium, Tritium
Explanation:
- A. Correct, hydrogen isotopes.
- **B. Different elements.
- **C. Different elements.
- **D. Isotopes of carbon.
Q65. Which property of elements is NOT affected by isotopes?
A. Chemical properties
B. Physical properties
C. Number of protons
D. Atomic number
✅ Correct Answer: A. Chemical properties
Explanation:
- A. Correct, isotopes have same Z → same chemical properties.
- **B. Physical properties differ (due to mass).
- **C. Protons same.
- **D. Atomic number same.
Q66. Isobars have:
A. Same mass number, different atomic number
B. Same atomic number, different mass number
C. Same number of protons
D. Same number of electrons
✅ Correct Answer: A. Same mass number, different atomic number
Explanation:
- A. Correct definition.
- **B. Wrong → isotopes.
- **C. Wrong.
- **D. Wrong.
Q67. Which pair is of isotopes?
A. ¹²C, ¹⁴C
B. ¹⁴C, ¹⁴N
C. ²³Na, ²³Mg
D. ⁴⁰Ca, ⁴⁰Ar
✅ Correct Answer: A. ¹²C, ¹⁴C
Explanation:
- A. Correct, both C with different mass.
- **B. Wrong, different elements.
- **C. Wrong, different elements.
- **D. Isobars.
Q68. Which pair is of isobars?
A. ¹⁴C, ¹²C
B. ⁴⁰K, ⁴⁰Ca
C. ²H, ³H
D. ²⁰Ne, ²²Ne
✅ Correct Answer: B. ⁴⁰K, ⁴⁰Ca
Explanation:
- **A. Isotopes of C.
- B. Correct, both have A=40, Z different.
- **C. Isotopes of H.
- **D. Isotopes of Ne.
Q69. Which isotope is used as a tracer in plants?
A. C-14
B. U-235
C. O-18
D. H-3
✅ Correct Answer: C. O-18
Explanation:
- **A. C-14 → dating.
- **B. U-235 → nuclear fuel.
- **C. Correct, O-18 used as tracer in photosynthesis studies.
- **D. H-3 (tritium) → radioactive hydrogen isotope.
Q70. Which hydrogen isotope is radioactive?
A. Protium (¹H)
B. Deuterium (²H)
C. Tritium (³H)
D. All are radioactive
✅ Correct Answer: C. Tritium (³H)
Explanation:
- **A. Protium stable.
- **B. Deuterium stable.
- **C. Correct, Tritium is radioactive.
- **D. Wrong.
Q71. Chlorine exists as isotopes ³⁵Cl (75%) and ³⁷Cl (25%). The average atomic mass is:
A. 35.0 u
B. 36.0 u
C. 35.5 u
D. 37.0 u
✅ Correct Answer: C. 35.5 u
Explanation:
- Avg mass = (35×0.75) + (37×0.25) = 35.5 u.
Q72. Atomic number of calcium is 20. Mass number is 40. Neutrons = ?
A. 20
B. 40
C. 60
D. 18
✅ Correct Answer: A. 20
Explanation:
- Neutrons = 40 – 20 = 20.
Q73. Which isotope of carbon is stable?
A. C-12
B. C-13
C. C-14
D. Both A and B
✅ Correct Answer: D. Both A and B
Explanation:
- **A. Stable.
- **B. Stable.
- **C. Radioactive.
- **D. Correct, C-12 and C-13 stable.
Q74. Which is true for isotopes of chlorine?
A. Same protons, different neutrons
B. Different protons, same neutrons
C. Same nucleons, different protons
D. Same electrons, different protons
✅ Correct Answer: A. Same protons, different neutrons
Explanation:
- Isotopes have same Z, different A → protons same, neutrons differ.
Q75. Which pair is NOT isobars?
A. ⁴⁰Ar, ⁴⁰K
B. ¹⁴C, ¹⁴N
C. ¹²C, ¹²N
D. ³⁷Cl, ³⁷Ar
✅ Correct Answer: C. ¹²C, ¹²N
Explanation:
- **A. Correct isobars (A=40).
- **B. Correct isobars (A=14).
- **C. Wrong, N-12 does not exist stably; not proper isobar example.
- **D. Correct isobars (A=37).
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