Nuclei MCQs: Radioactivity & Nuclear Reactions
Nuclei MCQs: Radioactivity & Nuclear Reactions
Class: CBSE Class 12 | Subject: Physics
Section 8: Atoms and Nuclei
Topic: Radioactivity & Nuclear Reactions
Based on CBSE Board Examination Pattern
Section 8: Atoms and Nuclei
Topic: Radioactivity & Nuclear Reactions
Based on CBSE Board Examination Pattern
Instructions: These Multiple Choice Questions (MCQs) are designed strictly as per the NCERT syllabus, making them ideal for CBSE Class 12 Board Examination preparation.
1. Radioactivity is a phenomenon in which:
Answer: B
Radioactivity is the spontaneous disintegration of unstable atomic nuclei accompanied by emission of α, β, or γ radiations. It is independent of temperature and pressure.
Radioactivity is the spontaneous disintegration of unstable atomic nuclei accompanied by emission of α, β, or γ radiations. It is independent of temperature and pressure.
2. The charge on an alpha particle is:
Answer: C
An alpha particle is a helium nucleus containing 2 protons and 2 neutrons, hence it carries a +2e charge.
An alpha particle is a helium nucleus containing 2 protons and 2 neutrons, hence it carries a +2e charge.
3. Gamma rays consist of:
Answer: D
Gamma rays are electromagnetic radiations of very high frequency emitted during nuclear de‑excitation.
Gamma rays are electromagnetic radiations of very high frequency emitted during nuclear de‑excitation.
4. The SI unit of radioactivity is:
Answer: B
1 Becquerel = 1 disintegration per second. Curie is an older unit.
1 Becquerel = 1 disintegration per second. Curie is an older unit.
5. Half‑life is the time required for:
Answer: C
Half‑life (T½) is the time during which the number of undecayed nuclei reduces to half its initial value.
Half‑life (T½) is the time during which the number of undecayed nuclei reduces to half its initial value.
6. During β⁻ decay, a nucleus emits:
Answer: B
In β⁻ decay, a neutron converts into a proton, emitting an electron and an antineutrino.
In β⁻ decay, a neutron converts into a proton, emitting an electron and an antineutrino.
7. Nuclear fission was first demonstrated by:
Answer: C
Otto Hahn and Fritz Strassmann discovered nuclear fission in uranium when bombarded with neutrons.
Otto Hahn and Fritz Strassmann discovered nuclear fission in uranium when bombarded with neutrons.
8. Moderator in a nuclear reactor is used to:
Answer: B
Moderators like heavy water or graphite slow fast neutrons to thermal energies for efficient fission.
Moderators like heavy water or graphite slow fast neutrons to thermal energies for efficient fission.
9. Binding energy per nucleon is highest for:
Answer: C
Iron (Fe‑56) has maximum binding energy per nucleon, hence most stable nucleus.
Iron (Fe‑56) has maximum binding energy per nucleon, hence most stable nucleus.
10. Nuclear fusion occurs at very high temperature because:
Answer: B
High temperature provides kinetic energy to overcome Coulomb repulsion between nuclei.
High temperature provides kinetic energy to overcome Coulomb repulsion between nuclei.
11. Positron emission is called:
Answer: A
β⁺ decay emits a positron when proton converts to neutron.
β⁺ decay emits a positron when proton converts to neutron.
12. Control rods are made of:
Answer: B
Cadmium absorbs excess neutrons controlling chain reaction.
Cadmium absorbs excess neutrons controlling chain reaction.
13. Activity is proportional to:
Answer: B
A = λN, hence proportional to undecayed nuclei.
A = λN, hence proportional to undecayed nuclei.
14. Fusion energy source of Sun is:
Answer: B
Hydrogen nuclei fuse into helium releasing vast energy.
Hydrogen nuclei fuse into helium releasing vast energy.
15. Mass defect represents:
Answer: B
Mass defect converts into binding energy via E = mc².
Mass defect converts into binding energy via E = mc².
16. Unit of binding energy:
Answer: D
Measured in Joule, eV, or MeV depending on scale.
Measured in Joule, eV, or MeV depending on scale.
17. Law of radioactive decay is:
Answer: B
N = N₀e⁻λt shows exponential decay.
N = N₀e⁻λt shows exponential decay.
18. Neutron was discovered by:
Answer: C
James Chadwick discovered neutron in 1932.
James Chadwick discovered neutron in 1932.
19. Chain reaction becomes uncontrolled in:
Answer: B
Uncontrolled fission releases explosive energy.
Uncontrolled fission releases explosive energy.
20. Energy released in nuclear reactions is due to:
Answer: B
Small mass converts into enormous energy (E = mc²).
Small mass converts into enormous energy (E = mc²).