Which electron configuration represents an atom in an excited state (i.e., an electron has been promoted to a higher-energy orbital while a lower-energy orbital is not filled as expected in the ground state)?

• A. $1s^{2}2s^{2}2p^{6}3s^1$
• B. $1s^{2}2s^2$
• C. $1s^{2}2s^{1}2p^1$
• D. $1s^{2}2s^{2}2p^1$

Answer: (C)

Verified step by step guidance

1. Understand that an atom in the ground state has electrons filling the lowest energy orbitals first, following the Aufbau principle, Hund's rule, and Pauli exclusion principle.
2. Write out the ground state electron configuration for the atom in question, ensuring all lower-energy orbitals are fully occupied before electrons occupy higher-energy orbitals.
3. Compare the given electron configurations to the ground state configuration to identify any that show electrons missing from a lower-energy orbital while higher-energy orbitals have electrons; this indicates an excited state.
4. Specifically, look for configurations where an electron has been 'promoted' from a filled lower-energy orbital (like 2s) to a higher-energy orbital (like 2p), leaving the lower orbital partially filled or less than expected.
5. Select the configuration that matches this pattern, confirming it represents an excited state because it violates the normal filling order by having electrons in higher orbitals while lower orbitals are not fully occupied.