Which of the following is an impossible electron configuration?

Question

BlackTom AI · Accepted Answer

The s subshell can hold a maximum of two electrons, so any configuration showing three electrons in an s orbital is impossible.

1) Relevant knowledge points and concepts
- Subshell capacities: s = 2 electrons, p = 6, d = 10, f = 14, determined by the number of magnetic orbitals (2l+1) times two spins.
- Pauli exclusion principle: no two electrons can have the same set of quantum numbers, limiting occupancy of a single orbital to two electrons with opposite spins.
- Aufbau principle and common ground-state configurations for elements (e.g., Si, Z = 14: 1s2 2s2 2p6 3s2 3p2).

2) Complete reasoning process step by step
- Read the proposed configuration: Si 1s2 2s2 2p6 3s3 3p1.
- Count electrons by subshell: 1s2 → 2, 2s2 → 2 (total 4), 2p6 → 6 (total 10), 3s3 → 3 (total 13), 3p1 → 1 (total 14). The total equals Z = 14, so electron count matches silicon, but distribution matters.
- Examine the 3s3 term: "3s" is an s subshell (l = 0) and consists of a single orbital, which can hold at most 2 electrons.

3) Intermediate steps / calculations
- Maximum electrons in 3s = 2 (1 orbital × 2 spins).
- Proposed occupancy 3 > 2 → violation.
- Therefore the configuration is not allowed by the Pauli exclusion principle.

4) Why the correct answer is correct
- The configuration Si 1s2 2s2 2p6 3s3 3p1 is impossible because it assigns three electrons to the single 3s orbital, exceeding the allowed maximum of two electrons per orbital.

5) Why other options (or correct ground-state alternatives) are not the answer
- The valid ground-state configuration for Si is 1s2 2s2 2p6 3s2 3p2, which respects subshell capacities and Aufbau ordering.
- No excited-state arrangement can place three electrons in a single s orbital; excited states still obey Pauli exclusion, so 3s3 remains impossible.

6) Conclusion
- The impossible configuration is Si 1s2 2s2 2p6 3s3 3p1, which matches the provided answer.

Which of the following is an impossible electron configuration?单项选择题

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