Which of the following (s) is (are) correct?

Options

A. The rest mass of a stable nucleus is less than the sum of the rest masses of its separated nucleons
B. The rest mass of a stable nucleus is greater than the sum of the rest masses of separatd nucleons
C. In nuclear fusion, energy is released by fusion of two nuclei of medium mass (approximately 100 amu)
D. In nuclear fission, energy is released by fragmentation of a very heavy nuceus

Answer

To solve the question, we need to analyze each statement regarding the properties of nuclei and the processes of nuclear fusion and fission.

### Step-by-Step Solution:

1. **Understanding the Rest Mass of a Nucleus**:
- The first statement claims that the rest mass of a stable nucleus is less than the sum of the rest masses of the separated nucleons.
- This is true because when nucleons (protons and neutrons) come together to form a nucleus, some mass is converted into binding energy, which is released. This phenomenon is described by Einstein's equation \(E=mc^2\).
- Therefore, the rest mass of the nucleus is indeed less than the sum of the rest masses of the individual nucleons.

**Conclusion**: Statement A is correct.

2. **Evaluating the Second Statement**:
- The second statement asserts that the rest mass of a stable nucleus is greater than the sum of the rest masses of separated nucleons.
- This directly contradicts the first statement. Since we established that the rest mass of a stable nucleus is less than that of separated nucleons, this statement is incorrect.

**Conclusion**: Statement B is incorrect.

3. **Analyzing Nuclear Fusion**:
- The third statement discusses nuclear fusion, stating that energy is released by the fusion of two nuclei of medium mass approximately 100 AMU.
- In nuclear fusion, light nuclei (typically less than 60 AMU) combine to form a heavier nucleus, releasing energy in the process.
- Therefore, the claim that fusion occurs with nuclei of medium mass (around 100 AMU) is incorrect.

**Conclusion**: Statement C is incorrect.

4. **Understanding Nuclear Fission**:
- The fourth statement claims that in nuclear fusion, energy is released by fragmentation of a very heavy nucleus.
- This describes nuclear fission, not fusion. In fission, a heavy nucleus splits into lighter nuclei, releasing energy. This statement is correct in the context of fission.

**Conclusion**: Statement D is correct.

### Final Answer:
The correct statements are A and D.

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