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Nuclei — NEET Physics PYQ MCQs with Solutions

Free NEET Physics PYQ Nuclei MCQs with step-by-step solutions covering Nucleus and Radioactivity, Nuclear Fission & Fusion and Binding Energy. Practise online on Prepizo — no login needed.

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Sample questions with solutions

Q1 — Nucleus and Radioactivity · easy · theory
What happens to the mass number and atomic number of an element when it emits $\gamma$-radiation?
A. Mass number decreases by four and atomic number decreases by two
B. Mass number and atomic number remain unchanged  ✓ Correct
C. Mass number remains unchanged, while atomic number decreases by one
D. Mass number increases by four and atomic number increases by two
Solution: $\gamma$-radiation is just a high-energy photon — its emission changes neither the atomic number nor the mass number.
Q2 — Nucleus and Radioactivity · easy · theory
$\alpha$-particle consists of
A. 2 electrons, 2 protons and 2 neutrons
B. 2 electrons and 4 protons only
C. 2 protons only
D. 2 protons and 2 neutrons only  ✓ Correct
Solution: An $\alpha$-particle is a doubly ionised helium nucleus (He²⁺) — 2 protons and 2 neutrons, with no electrons.
Q3 — Nucleus and Radioactivity · easy · numerical
The half-life of radium is about 1600 yr. Of 100 g of radium existing now, 25 g will remain unchanged after
A. 4800 yr
B. 6400 yr
C. 2400 yr
D. 3200 yr  ✓ Correct
Solution: $\frac{25}{100} = \frac{1}{4} = \left(\frac{1}{2}\right)^2$ — 2 half-lives $t = 2 \times 1600 = 3200$ yr
Q4 — Nucleus and Radioactivity · easy · theory
A nuclear reaction given by $^{A}_{Z}X \rightarrow\ ^{A}_{Z+1}Y +\ ^{0}_{-1}e + \bar{\nu}$ represents
A. fusion
B. fission
C. $\beta$-decay  ✓ Correct
D. $\gamma$-decay
Solution: Emission of an electron ($^{0}_{-1}e$) and an antineutrino with Z increasing by 1 is $\beta^-$ decay.
Q5 — Nucleus and Radioactivity · easy · theory
The mass number of a nucleus is
A. sometimes equal to its atomic number  ✓ Correct
B. sometimes less than and sometimes more than its atomic number
C. always less than its atomic number
D. always more than its atomic number
Solution: Mass number = protons + neutrons. For ordinary hydrogen (no neutrons) the mass number equals the atomic number; otherwise it is greater. So it is sometimes equal.
Q6 — Nucleus and Radioactivity · easy · numerical
Half-life of a radioactive substance is 12.5 h and its mass is 256 g. After what time, the amount of remaining substance is 1 g?
A. 75 h
B. 100 h  ✓ Correct
C. 125 h
D. 150 h
Solution: $\frac{1}{256} = \left(\frac{1}{2}\right)^8$ — 8 half-lives $t = 8 \times 12.5 = 100$ h
Q7 — Nucleus and Radioactivity · easy · numerical
Half-life period of a radioactive substance is 6 h. After 24 h activity is 0.01 µC, what was the initial activity?
A. 0.04 µC
B. 0.08 µC
C. 0.24 µC
D. 0.16 µC  ✓ Correct
Solution: 24 h = 4 half-lives $R_0 = 0.01 \times 2^4 = 0.16$ µC
Q8 — Nucleus and Radioactivity · easy · theory
Which of the following is positively charged?
A. $\alpha$-particle  ✓ Correct
B. $\beta$-particle
C. $\gamma$-rays
D. X-rays
Solution: X-rays and $\gamma$-rays are electromagnetic waves (no charge); $\beta$-particles are negatively charged electrons. The $\alpha$-particle (helium nucleus) is positively charged.
Q9 — Nucleus and Radioactivity · easy · numerical
The half-life of a radioactive material is 3 h. If the initial amount is 300 g, then after 18 h, it will remain
A. 4.68 g  ✓ Correct
B. 46.8 g
C. 9.375 g
D. 93.75 g
Solution: 18 h = 6 half-lives $N = 300 \times \frac{1}{64} = 4.68$ g
Q10 — Nucleus and Radioactivity · easy · theory
The relationship between disintegration constant ($\lambda$) and half-life (T) will be
A. $\lambda = \frac{\log_{10} 2}{T}$
B. $\lambda = \frac{\log_e 2}{T}$  ✓ Correct
C. $\lambda = \frac{T}{\log_e 2}$
D. $\lambda = \frac{\log_2 e}{T}$
Solution: From $0.5N_0 = N_0e^{-\lambda T}$: $\lambda T = \log_e 2$ $\lambda = \frac{\log_e 2}{T}$
Q11 — Nucleus and Radioactivity · easy · theory
Alpha particles are
A. 2 free protons
B. helium atoms
C. singly ionised helium atoms
D. doubly ionised helium atoms  ✓ Correct
Solution: An $\alpha$-particle is a helium nucleus — a helium atom stripped of both its electrons, i.e. a doubly ionised helium atom.
Q12 — Nucleus and Radioactivity · easy · theory
A free neutron decays into a proton, an electron and
A. a beta particle
B. an alpha particle
C. an antineutrino  ✓ Correct
D. a neutrino
Solution: $^{1}_{0}n \rightarrow\ ^{1}_{1}H + \beta^- + \bar{\nu}$ A free neutron decays into a proton, an electron and an antineutrino.
Q13 — Nucleus and Radioactivity · easy · theory
The most penetrating radiation out of the following is
A. $\gamma$-rays  ✓ Correct
B. $\alpha$-particles
C. $\beta$-rays
D. X-rays
Solution: Penetrating power increases with photon energy ($\propto \frac{1}{\lambda}$). $\gamma$-rays have the shortest wavelength, hence the maximum penetrating power.
Q14 — Nucleus and Radioactivity · easy · numerical
The mass number of He is 4 and that for sulphur is 32. The radius of sulphur nuclei is larger than that of helium by
A. $\sqrt{8}$
B. 4
C. 2  ✓ Correct
D. 8
Solution: $R \propto A^{1/3}$ $\frac{R_S}{R_{He}} = \left(\frac{32}{4}\right)^{1/3} = 2$
Q15 — Nucleus and Radioactivity · easy · theory
If the nuclear force between two protons, two neutrons and between proton and neutron is denoted by $F_{pp}$, $F_{nn}$ and $F_{pn}$ respectively, then
A. $F_{pp} \approx F_{nn} \approx F_{pn}$
B. $F_{pp} \neq F_{nn}$ and $F_{pp} = F_{nn}$
C. $F_{pp} = F_{nn} = F_{pn}$  ✓ Correct
D. $F_{pp} \neq F_{nn} \neq F_{pn}$
Solution: Nuclear forces are charge independent — they act between n–n, p–p and n–p pairs with the same strength.
Q16 — Nucleus and Radioactivity · easy · numerical
In the nucleus of $_{11}$Na²³, the number of protons, neutrons and electrons are
A. 11, 12, 0  ✓ Correct
B. 23, 12, 11
C. 12, 11, 0
D. 23, 11, 12
Solution: Protons $= Z = 11$; neutrons $= A - Z = 23 - 11 = 12$; there are no electrons inside the nucleus.
Q17 — Nucleus and Radioactivity · easy · numerical
The half-life of radium is 1600 yr. The fraction of a sample of radium that would remain after 6400 yr is
A. $\frac{1}{4}$
B. $\frac{1}{2}$
C. $\frac{1}{8}$
D. $\frac{1}{16}$  ✓ Correct
Solution: $n = \frac{6400}{1600} = 4$ half-lives $\frac{N}{N_0} = \left(\frac{1}{2}\right)^4 = \frac{1}{16}$
Q18 — Nucleus and Radioactivity · easy · theory
The constituents of atomic nuclei are believed to be
A. neutrons and protons  ✓ Correct
B. protons only
C. electrons and protons
D. electrons, protons and neutrons
Solution: A nucleus of mass number A and atomic number Z contains Z protons and (A − Z) neutrons — nucleons only.
Q19 — Nucleus and Radioactivity · easy · theory
Which of the following statements is true for nuclear forces?
A. They obey the inverse square law of distance
B. They obey the inverse third power law of distance
C. They are short range forces  ✓ Correct
D. They are equal in strength to electromagnetic forces
Solution: Nuclear forces are short-range (a few fermi), charge-independent, non-central and are the strongest forces in nature — about 100 times the electrostatic force.
Q20 — Nucleus and Radioactivity · easy · numerical
A radioactive element has half-life period 800 yr. After 6400 yr, what amount will remain?
A. $\frac{1}{2}$
B. $\frac{1}{16}$
C. $\frac{1}{8}$
D. $\frac{1}{256}$  ✓ Correct
Solution: $n = \frac{6400}{800} = 8$ half-lives $\frac{N}{N_0} = \left(\frac{1}{2}\right)^8 = \frac{1}{256}$
Q21 — Nucleus and Radioactivity · easy · numerical
The nucleus $^{115}_{48}$Cd, after two successive $\beta^-$-decay will give
A. $^{115}_{46}$Pa
B. $^{114}_{49}$In
C. $^{113}_{50}$Sn
D. $^{115}_{50}$Sn  ✓ Correct
Solution: Each $\beta^-$ decay keeps A the same and raises Z by 1: $48 \rightarrow 49 \rightarrow 50$, giving $^{115}_{50}$Sn.
Q22 — Nucleus and Radioactivity · easy · numerical
A radioactive sample with a half-life of 1 month has the label: 'Activity = 2 microcurie on 1-8-1991'. What would be its activity two months earlier?
A. 1.0 microcurie
B. 0.5 microcurie
C. 4 microcurie
D. 8 microcurie  ✓ Correct
Solution: Two months = 2 half-lives. Going backwards the activity doubles per half-life: $2 \times 2^2 = 8$ microcurie
Q23 — Nuclear Fission & Fusion and Binding Energy · easy · numerical
The energy equivalent of 0.5 g of a substance is
A. $4.5 \times 10^{13}$ J  ✓ Correct
B. $1.5 \times 10^{13}$ J
C. $0.5 \times 10^{13}$ J
D. $4.5 \times 10^{16}$ J
Solution: $E = mc^2 = 0.5 \times 10^{-3} \times (3 \times 10^8)^2 = 4.5 \times 10^{13}$ J
Q24 — Nuclear Fission & Fusion and Binding Energy · easy · theory
Fusion reaction takes place at high temperature because
A. atoms get ionised at high temperature
B. kinetic energy is high enough to overcome the coulomb repulsion between nuclei  ✓ Correct
C. molecules break up at high temperature
D. nuclei break up at high temperature
Solution: At high temperature the kinetic energy of the nuclei is high enough to overcome the Coulomb repulsion between them, allowing fusion.
Q25 — Nuclear Fission & Fusion and Binding Energy · easy · theory
If in a nuclear fusion process, the masses of the fusing nuclei be $m_1$ and $m_2$ and the mass of the resultant nucleus be $m_3$, then
A. $m_3 = m_1 + m_2$
B. $m_3 = |m_1 - m_2|$
C. $m_3 < (m_1 + m_2)$  ✓ Correct
D. $m_3 > (m_1 + m_2)$
Solution: In fusion some mass (the mass defect) is converted into released energy, so $m_3 < m_1 + m_2$.
Q26 — Nuclear Fission & Fusion and Binding Energy · easy · theory
Solar energy is mainly caused due to
A. fusion of protons during synthesis of heavier elements  ✓ Correct
B. gravitational contraction
C. burning of hydrogen in the oxygen
D. fission of uranium present in the sun
Solution: In the sun, energy is produced by the fusion of four protons (hydrogen nuclei) into a helium nucleus, releasing enormous energy.
Q27 — Nuclear Fission & Fusion and Binding Energy · easy · theory
Which of the following are suitable for the fusion process?
A. Light nuclei  ✓ Correct
B. Heavy nuclei
C. Elements lying in the middle of periodic table
D. Elements lying in the middle of binding energy curve
Solution: Light nuclei (A < 20) have relatively small binding energy per nucleon; combining them into a heavier nucleus raises it and liberates a large amount of energy.
Q28 — Nuclear Fission & Fusion and Binding Energy · easy · theory
$m_p$ and $m_n$ are masses of proton and neutron respectively. An element of mass m has Z protons and N neutrons, then
A. $m > Zm_p + Nm_n$
B. $m = Zm_p + Nm_n$
C. $m < Zm_p + Nm_n$  ✓ Correct
D. m may be greater than, less than or equal to $Zm_p + Nm_n$, depending on nature of element
Solution: The nuclear mass is always slightly less than the sum of the masses of its constituent nucleons — the difference is the mass defect (binding energy).
Q29 — Nuclear Fission & Fusion and Binding Energy · easy · theory
Which of the following is used as a moderator in nuclear reactors?
A. Plutonium
B. Cadmium
C. Heavy water  ✓ Correct
D. Uranium
Solution: Moderators slow down fast neutrons. Heavy water, graphite and beryllium oxide are used; heavy water is the best moderator. (Cadmium is used for control rods.)
Q30 — Nuclear Fission & Fusion and Binding Energy · easy · theory
Heavy water is used as a moderator in a nuclear reactor. The function of the moderator is
A. to control energy released in the reactor
B. to absorb neutrons and stop chain reaction
C. to cool the reactor
D. to slow down the neutrons to thermal energies  ✓ Correct
Solution: The moderator slows the fast secondary neutrons produced in fission down to thermal energies, since fission is initiated efficiently only by slow neutrons.