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 two statements 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 occupy different energy levels and sub-shells. The presence of multiple electrons leads to electron-electron repulsions, which affects the energy levels of the orbitals.

**Hint**: Remember that in multi-electron atoms, interactions between electrons play a significant role in determining energy levels.

2. **Energy Levels and Sub-Shells**:
- Each sub-shell (s, p, d, f) has a different azimuthal quantum number (l). For example, s has l = 0, p has l = 1, d has l = 2, and f has l = 3. The energy of an orbital increases with increasing l for a given principal quantum number (n).

**Hint**: Recall the values of l for different sub-shells and how they influence energy.

3. **n + l Rule**:
- The n + l rule states that the energy of an orbital is determined by the sum of its principal quantum number (n) and its azimuthal quantum number (l). The lower the n + l value, the lower the energy of the orbital. If two orbitals have the same n + l value, the one with the lower n will have lower energy.

**Hint**: Understand how to apply the n + l rule to compare the energies of different orbitals.

4. **Evaluating the Assertion and Reason**:
- The assertion (A) is true because in multi-electron atoms, different sub-shells indeed have different energies due to the varying values of l.
- The reason (R) is also true because the energy of an orbital does depend on the n + l value. The n + l rule provides a method to compare the energies of different orbitals.

**Hint**: Check if both statements are true and if one explains the other.

5. **Conclusion**:
- Since both the assertion and reason are true, and the reason provides a correct explanation for the assertion, we conclude that the correct option is that both statements are true and the reason is the correct explanation of the assertion.

**Final Answer**: Option 1st is correct: Both assertion and reason are true, and the reason is the correct explanation of the assertion.