which must represent an atom in an excited state?

Options

A. `1s^(2),2s^(2)2p^(1)`
B. `1s^(2),2s^(2)2p^(2)`
C. `1s^(2),2s^(2)2p^(2),3s^(1)`
D. `1s^(2),2s^(2)2p^(5)`

Explanation

To determine which atom is in an excited state, we need to analyze the given electron configurations step by step.

### Step-by-Step Solution:

1. **Understanding Electron Configurations**:
- Electron configurations represent the distribution of electrons in an atom's orbitals. The Aufbau principle states that electrons fill orbitals starting from the lowest energy level to the highest.

2. **Analyzing the Given Configurations**:
- **Configuration 1**: 1s² 2s² 2p¹
- This configuration fills the 1s and 2s orbitals completely and has one electron in the 2p orbital. This follows the Aufbau principle and is in the ground state.

- **Configuration 2**: 1s² 2s² 2p²
- Similar to the first configuration, this configuration fills the 1s and 2s orbitals completely and has two electrons in the 2p orbital. This is also in the ground state.

- **Configuration 3**: 1s² 2s² 2p² 3s¹
- Here, the 1s and 2s orbitals are filled, and there are two electrons in the 2p orbital. However, there is also one electron in the 3s orbital. The 2p orbital can hold up to 6 electrons, and since it only has 2 electrons, the atom has moved one electron from the 2p to the 3s orbital. This indicates that the atom is in an excited state.

- **Configuration 4**: 1s² 2s² 2p⁵
- This configuration has completely filled the 1s, 2s, and 2p orbitals, following the Aufbau principle. Thus, this is also in the ground state.

3. **Conclusion**:
- The only configuration that indicates an atom in an excited state is **Configuration 3**: 1s² 2s² 2p² 3s¹, as it shows an electron has jumped to a higher energy level while the lower energy levels are not completely filled.

### Final Answer:
The atom represented by the configuration **1s² 2s² 2p² 3s¹** is in an excited state.

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