Physics: Nuclei Class 12 MCQ

1. The total number of protons and neutrons in a nucleus is called: a) Atomic number b) Atomic weight c) Mass number d) Nuclear force Answer: c) Mass number Explanation: The mass number is the sum of the number of protons and neutrons in the nucleus. 2. The force that binds protons and neutrons in the nucleus is: a) Gravitational force b) Electromagnetic force c) Nuclear force d) Weak force Answer: c) Nuclear force Explanation: The nuclear force is a strong force that acts between protons and neutrons, holding them together inside the nucleus. 3. The phenomenon of a nucleus emitting an alpha particle is known as: a) Fission b) Fusion c) Alpha decay d) Beta decay Answer: c) Alpha decay Explanation: Alpha decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle. 5. The half-life of a radioactive substance is: a) The time taken for its complete decay b) Half the time taken for its complete decay c) The time taken for half of its atoms to decay d) Twice the time taken for its complete decay Answer: c) The time taken for half of its atoms to decay Explanation: The half-life is the time required for half the atoms in a sample to decay. 6. Energy released in nuclear reactions is due to: a) Conversion of protons to neutrons b) Conversion of electrons to protons c) Conversion of a small amount of mass into energy d) Bond breaking in atoms Answer: c) Conversion of a small amount of mass into energy Explanation: The energy in nuclear reactions comes from the conversion of a small amount of mass into energy, as per Einstein’s mass-energy equivalence principle. 7. Which of the following is a unit for radioactivity? a) Joule b) Becquerel c) Watt d) Coulomb Answer: b) Becquerel Explanation: Becquerel is a unit of radioactivity which represents one decay per second. 8. The process in which two light nuclei combine to form a heavier nucleus is called: a) Fission b) Fusion c) Decay d) Separation Answer: b) Fusion Explanation: Fusion is the process where two light nuclei combine to produce a heavier nucleus, releasing energy. 9. The number of neutrons in the nucleus can be determined by: a) Mass number – Atomic number b) Mass number + Atomic number c) Mass number / Atomic number d) Atomic number – Mass number Answer: a) Mass number – Atomic number Explanation: Neutrons = Mass number (protons + neutrons) – Atomic number (protons). 10. A nucleus emits a beta particle. This means it: a) Gains a proton b) Loses a proton c) Loses a neutron d) Gains a neutron Answer: a) Gains a proton Explanation: In beta decay, a neutron decays into a proton and emits a beta particle. 11. Nuclear reactors typically use which isotope as fuel? a) Hydrogen-1 b) Carbon-12 c) Uranium-235 d) Oxygen-16 Answer: c) Uranium-235 Explanation: Uranium-235 is commonly used as fuel in nuclear reactors due to its fissile nature. 12. A gamma ray is: a) A charged particle b) A type of X-ray c) Electromagnetic radiation d) A neutron Answer: c) Electromagnetic radiation Explanation: Gamma rays are electromagnetic radiation, similar to X-rays but of higher energy. 13. Neutrinos are released in: a) Alpha decay b) Gamma decay c) Beta decay d) Fusion Answer: c) Beta decay Explanation: During beta decay, along with beta particles, neutrinos or antineutrinos are also released. 14. Which of the following forces is the weakest? a) Gravitational force b) Electromagnetic force c) Weak nuclear force d) Strong nuclear force Answer: a) Gravitational force Explanation: Among the fundamental forces, gravitational force is the weakest. 15. Nucleons refer to: a) Only protons b) Only neutrons c) Protons and electrons d) Protons and neutrons Answer: d) Protons and neutrons Explanation: Nucleons are particles found in the nucleus, which include both protons and neutrons.