Assertion (A) : The configuration of B atom cannot be `1s^(2)2s^(3)`
Reason (R ) : Hund's rule demands that the configuration should display maximum multiplicity

Options

A. If both (A) and (R ) correct and (R ) is the correct explanation for (A)
B. If both (A) and (R ) correct and (R ) is the correct explanation for (A)
C. If (A) is correct but (R ) is incorrect
D. If (A) is incorrect but (R ) is correct

Explanation

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

### Step 1: Analyze the Assertion (A)
The assertion states that the configuration of a Boron (B) atom cannot be `1s^(2)2s^(3)`.

- **Understanding Boron**: Boron has an atomic number of 5, which means it has 5 electrons.
- **Correct Electron Configuration**: The correct electron configuration for Boron is `1s^(2) 2s^(2) 2p^(1)`. This means:
- The 1s subshell can hold 2 electrons (which it does).
- The 2s subshell can also hold 2 electrons (which it does).
- The 2p subshell can hold 6 electrons, but for Boron, it only has 1 electron in this subshell.

Thus, the configuration `1s^(2) 2s^(3)` is incorrect because the 2s subshell can only hold a maximum of 2 electrons.

### Step 2: Analyze the Reason (R)
The reason states that Hund's rule demands that the configuration should display maximum multiplicity.

- **Understanding Hund's Rule**: Hund's rule states that electrons will fill degenerate orbitals (orbitals of the same energy) singly first before pairing up. This maximizes the total spin, leading to maximum multiplicity.
- **Applicability to Boron**: While Hund's rule is relevant for filling orbitals, it does not directly explain why the assertion about the incorrect configuration is true. The assertion is primarily incorrect due to the Pauli exclusion principle, which states that no two electrons can have the same set of quantum numbers in an atom.

### Conclusion
- **Assertion (A)** is true: The configuration `1s^(2) 2s^(3)` is indeed incorrect for Boron.
- **Reason (R)** is true: Hund's rule does state that configurations should maximize multiplicity.
- However, **R is not the correct explanation for A** because the reason for the incorrectness of the assertion is due to the Pauli exclusion principle, not Hund's rule.

### Final Answer
Both A and R are correct, but R is not the correct explanation for A.

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