Which of the following valence electron configurations corresponds to an atom in an excited state?

• A. 2s2 2p4
• B. 2s2 2p5
• C. 2s2 2p6
• D. 2s1 2p6

Answer: (D)

Verified step by step guidance

1. Understand that an atom in its ground state has electrons filling the lowest available energy orbitals according to the Aufbau principle, Hund's rule, and the Pauli exclusion principle.
2. Review the given electron configurations and identify which ones follow the expected filling order for the 2s and 2p orbitals in the second energy level.
3. Recognize that the ground state configuration for an element with six valence electrons (like oxygen) is 2s2 2p4, where the 2s orbital is fully occupied before electrons fill the 2p orbitals.
4. Note that the configuration 2s1 2p6 shows an electron promoted from the 2s orbital to the 2p orbital, which is not the lowest energy arrangement and thus represents an excited state.
5. Conclude that the configuration 2s1 2p6 corresponds to an excited state because it violates the normal filling order by having fewer electrons in the 2s orbital and more in the 2p orbital.