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 represents an excited state, we need to analyze the given electronic configurations and identify any deviations from the expected ground state configurations.

### Step-by-Step Solution:

1. **Understand Ground State Configuration**:
- The ground state electronic configuration of an atom is the arrangement of electrons in the lowest energy levels. For example, the ground state configuration for atomic number 5 (Boron) is 1s² 2s² 2p¹, and for atomic number 6 (Carbon) it is 1s² 2s² 2p².

2. **Examine the Options**:
- The question provides several configurations. We need to identify which one shows an electron in a higher energy level than expected.

3. **Identify the Excited State**:
- In an excited state, one or more electrons have absorbed energy and moved to a higher energy orbital than they would in the ground state.
- For example, if we see an electron in the 3s subshell when it should be in the 2p subshell, this indicates that the electron has been excited.

4. **Analyze the Given Configuration**:
- The configuration mentioned in the transcript indicates that there are two electrons in the 2p subshell and an additional electron in the 3s subshell.
- This is unusual because after filling the 2p subshell (which can hold a maximum of 6 electrons), the next electron should fill the 3p subshell, not the 3s subshell.

5. **Conclusion**:
- Since the configuration shows an electron in the 3s subshell instead of the expected 3p, we conclude that this represents an excited state of the atom.

### Final Answer:
The configuration that must represent an atom in an excited state is the one with an electron in the 3s subshell while the 2p subshell is already filled.

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