Physics · Atoms and nuclei · chapter 13 of 14
Class 12 Physics Chapter 13: Nuclei MCQs with answers
10 multiple-choice questions on Nuclei, each with its answer and the reason, written from the NCERT chapter. For the CBSE board, JEE Main and NEET, which are set from the same NCERT chapter. Open a question's solution to check your working; then take the free test to find your next chapter.
Question 1
Which of the following nuclides is an isotone of 146C?
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Answer: (d) 168O
Isotones have the same neutron number N = A − Z. For 146C, N = 8; for 168O, N = 8. 147N (N = 7) is an isobar of 146C, 136C (N = 7) an isotope, and 158O has N = 7.
Question 2
The radius of a nucleus is measured to be 3.6 fm. Taking R₀ = 1.2 fm in R = R₀A1/3, its mass number is
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Answer: (a) 27
A1/3 = R/R₀ = 3.6/1.2 = 3.0, so A = 3.0³ = 27. 3 forgets to cube; 9 squares in place of cubing; 81 is 3⁴.
Question 3
When beryllium was bombarded with α-particles, it gave out a neutral radiation that could knock protons out of light nuclei. Chadwick concluded that this radiation was not made of photons because
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Answer: (a) photons able to do this would need far more energy than the bombardment could supply
To knock protons out, photons would have needed far more energy than the α-bombardment could provide. Chadwick therefore proposed new neutral particles, neutrons, of mass very nearly that of the proton, which can transfer energy to protons efficiently in collisions.
Question 4
The radius of nucleus X is twice the radius of nucleus Y. The ratio of their mass numbers AX : AY is
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Answer: (c) 8
A ∝ R³, so AX/AY = 2³ = 8. 2 treats A as proportional to R, 4 to R²; 1.26 = 21/3 applies the relation the wrong way round.
Question 5
The radius of a nucleus is about 10⁻⁴ times the radius of its atom. The fraction of the atom's volume occupied by the nucleus is about
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Answer: (a) 10⁻¹²
Volume ∝ r³, so the fraction is (10⁻⁴)³ = 10⁻¹²: the atom is almost empty, though the nucleus holds more than 99.9% of its mass.
Question 6
The binding energy of the 42He nucleus is 28.3 MeV. Its mass defect is (take 1 u = 931.5 MeV/c²)
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Answer: (b) 0.0304 u
Eb = ΔMc², so ΔM = 28.3 MeV ÷ 931.5 MeV/u = 0.0304 u. 0.0076 u is the mass defect per nucleon (÷ 4); 0.0152 u divides by 2; 0.304 u slips a decimal.
Question 7
A power station sends out 3.6 × 10¹⁶ J of electrical energy in a year. The mass equivalent of this energy is (c = 3 × 10⁸ m s⁻¹)
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Answer: (c) 0.40 kg
m = E/c² = 3.6 × 10¹⁶ / (9 × 10¹⁶) kg = 0.40 kg. Dividing by c alone gives 1.2 × 10⁸ kg.
Question 8
Of the following nuclei, the one in which a nucleon is, on average, most tightly bound is
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Answer: (d) 5626Fe
The binding energy per nucleon has its maximum, about 8.75 MeV, near A = 56; it is lower for light nuclei (⁴He about 7.1 MeV, ¹²C about 7.7 MeV) and for heavy ones (²³⁸U about 7.6 MeV).
Question 9
Which of the following is NOT a property of the nuclear force between two nucleons?
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Answer: (a) It depends on whether the nucleons carry electric charge.
The nuclear force is nearly the same for n–n, p–n and p–p pairs: it does not depend on electric charge. The other three are its properties: very strong, short-ranged, and acting between any two nucleons.
Question 10
The potential energy of a pair of nucleons has its minimum at a separation of about 0.8 fm. At a separation of 0.5 fm the force between them is
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Answer: (a) strongly repulsive
Below the minimum (r < 0.8 fm) the potential energy rises steeply as r decreases, so the force is strongly repulsive; it is attractive beyond 0.8 fm and falls to zero beyond a few fm.
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