A : in a multielectron atom , the electrons in different sub-shell have different energies
R : energy of an orbital depends upon n+l value .

Options

A. IF both assertion & reason are true and the reason is the correct explanation of the assertion then mark
B. IF both assertion & reason are true but the reason is not the correct explanation of the assertion , then mark
C. IF assertion is true statement but reason is false , then mark
D. If both assertion and reason are false statements then mark

Answer

To solve the question, we need to analyze the assertion (A) and reason (R) provided:

**Assertion (A):** In a multi-electron atom, the electrons in different sub-shells have different energies.
**Reason (R):** The energy of an orbital depends upon the n + l value.

### Step-by-Step Solution:

1. **Understanding Multi-electron Atoms:**
- In multi-electron atoms, electrons are arranged in various energy levels and sub-shells. Each sub-shell corresponds to a different quantum state.

2. **Energy Levels in Multi-electron Atoms:**
- Unlike hydrogen-like atoms, where energy depends only on the principal quantum number (n), in multi-electron atoms, the energy of electrons also depends on the type of sub-shell they occupy (s, p, d, f).

3. **n + l Rule:**
- The n + l rule states that the energy of an electron in an orbital can be determined by the sum of the principal quantum number (n) and the azimuthal quantum number (l).
- For example, for the 3s orbital, n = 3 and l = 0, so n + l = 3. For the 3p orbital, n = 3 and l = 1, so n + l = 4. For the 4s orbital, n = 4 and l = 0, so n + l = 4.

4. **Comparison of Energies:**
- When comparing orbitals, if two orbitals have the same n + l value, the one with the lower n value will generally have lower energy. Thus, different sub-shells (with different l values) will have different energies.

5. **Conclusion:**
- The assertion (A) is correct because electrons in different sub-shells do indeed have different energies due to their different n + l values.
- The reason (R) is also correct as it explains why the assertion holds true. The energy of an orbital does depend on the n + l value.

### Final Answer:
Both the assertion (A) and reason (R) are correct, and the reason is the correct explanation of the assertion.