Choice C is correct because:
- In nuclear processes, the number of neutrons is not always conserved. Neutrons can be converted into protons, and vice versa, through processes like beta decay.
- For example, in beta minus decay, a neutron decays into a proton, an electron, and an antineutrino: n→p+e−+νˉe. In this process, the number of neutrons decreases by one, while the number of protons increases by one.
- Similarly, in beta plus decay, a proton decays into a neutron, a positron, and a neutrino: p→n+e++νe. In this process, the number of protons decreases by one, while the number of neutrons increases by one.
- Therefore, the total number of neutrons is not conserved in nuclear processes.
Choice A is incorrect because:
- Charge is always conserved in nuclear processes. The total charge before a reaction must equal the total charge after the reaction.
- For example, in beta minus decay (n→p+e−+νˉe), the neutron has a charge of 0, and the proton has a charge of +1, the electron has a charge of -1, and the antineutrino has a charge of 0. The total charge before the decay is 0, and the total charge after the decay is +1 + (-1) + 0 = 0.
- Therefore, charge is conserved.
Choice B is incorrect because:
- Momentum is always conserved in nuclear processes. This is a fundamental law of physics.
- The total momentum before a reaction must equal the total momentum after the reaction, considering all particles involved.
Choice D is incorrect because:
- The total number of nucleons (protons and neutrons) is conserved in nuclear processes. Although neutrons and protons can transform into each other, the total number of nucleons remains constant.
- For example, in beta decay, a neutron transforms into a proton, or vice versa, but the total number of nucleons does not change.
- Therefore, the total number of nucleons is conserved.

Particle and Nuclear Physics | 2010 - M/J - 9702/11 | Q39

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