Which of the following represents an excited state of an atom?

Options

A. `[Ne]3s^(2)3p^(6)4s^(2)3d^(8)`
B. `[Ne]3s^(2)3p^(6)4s^(1)3d^(5)`
C. `[Ne]3s^(2)3p^(6)4s^(2)3d^(1)`
D. `1s^(2)2s^(2)2p^(5)3s^(1)`

Answer

To determine which option represents an excited state of an atom, we need to analyze the electron configurations provided in each option. An excited state occurs when one or more electrons have absorbed energy and moved to a higher energy level or subshell, rather than being in the lowest possible energy configuration (ground state).

### Step-by-Step Solution:

1. **Understand Electron Configuration**:
- The electron configuration of an atom describes the distribution of electrons in atomic orbitals. Electrons fill orbitals starting from the lowest energy level to the highest (Aufbau principle).

2. **Identify Ground State vs. Excited State**:
- Ground state: Electrons are in the lowest energy configuration.
- Excited state: One or more electrons have moved to a higher energy level or subshell.

3. **Analyze Each Option**:
- **Option 1**: Configuration is 3s² 3p⁶ 4s² 3d⁴.
- This configuration shows fully filled s and p orbitals, and the d subshell is partially filled. This is a ground state configuration.

- **Option 2**: Configuration is 3s² 3p⁶ 4s¹ 3d⁵.
- Here, the 3d subshell is half-filled (3d⁵), which is a stable configuration, but it does not indicate an excited state since the 4s subshell has only one electron. This is also a ground state configuration.

- **Option 3**: Configuration is 3s² 3p⁶ 4s² 3d¹.
- Similar to option 1, this configuration shows fully filled s and p orbitals, with only one electron in the d subshell. This is a ground state configuration.

- **Option 4**: Configuration is 2p⁵ 3s¹.
- In this configuration, the 2p subshell is not fully filled (it can hold up to 6 electrons), and one electron from the 3s subshell has moved to the 2p subshell. This indicates that an electron has absorbed energy and moved to a higher energy level, representing an excited state.

4. **Conclusion**:
- Based on the analysis, **Option 4** represents an excited state of the atom.

### Final Answer:
**Option 4** represents an excited state of an atom.