In which of the following electron distribution in ground state, only the Hund's rule is violated?

Options

A.
B.
C.
D.

Explanation

To determine which electron distribution violates only Hund's rule, we need to understand the basics of electron configuration and the rules governing it:

1. **Hund's Rule**: This rule states that when electrons occupy degenerate orbitals (orbitals of the same energy), one electron enters each orbital singly before any orbital gets a second electron. Additionally, all singly occupied orbitals must have electrons with the same spin.

2. **Pauli Exclusion Principle**: This principle states that no two electrons in an atom can have the same set of four quantum numbers. Therefore, an orbital can hold a maximum of two electrons, and they must have opposite spins.

3. **Aufbau Principle**: This principle states that electrons fill orbitals starting from the lowest energy level to the highest.

Now, let's analyze the options provided in the question step-by-step:

### Step 1: Analyze Each Option
- **Option A**:
- Configuration: 2s² 2p²
- Analysis: The 2s orbital is filled with 2 electrons. Then, in the 2p orbitals, both electrons are filled in the same orbital before occupying the others. This violates Hund's rule because the electrons should fill each of the 2p orbitals singly before pairing.

- **Option B**:
- Configuration: 2s² 2p²
- Analysis: If the 2p orbital is filled before the 2s orbital is fully filled, it violates the Aufbau principle. Therefore, this option is not correct.

- **Option C**:
- Configuration: 2s² 2p³
- Analysis: In this case, the 2s orbital is filled with 2 electrons, but if they are paired with the same spin, it violates the Pauli Exclusion Principle. Additionally, if the 2p orbitals are filled with 3 electrons in the same spin, it violates both Hund's rule and the Pauli Exclusion Principle.

- **Option D**:
- Configuration: 2s² 2p³
- Analysis: If the 2s orbital is filled with 2 electrons in opposite spins and then the 2p orbitals are filled with 3 electrons in the same spin, this violates Hund's rule but does not violate the Pauli Exclusion Principle.

### Conclusion
From the analysis, we find that:
- **Option A** is the only option where only Hund's rule is violated.

### Final Answer
The correct answer is **Option A**.