Which of the following electron configurations represents that of an atom in its ground state?

Analyze the question: The ground state is the lowest-energy arrangement of electrons. Electrons fill orbitals in order of increasing energy (Aufbau principle), so no electron should be in a higher subshell while a lower one has space.

Concept: The filling order is 1s → 2s → 2p → 3s → 3p. A lower subshell must be full before electrons go into the next one.

Check each option (the exponents are partly garbled in the question, so I read them in the natural way):

• A. 1s²2s¹2p¹: Here 2s has only 1 electron while an electron sits in 2p. This is an excited state, because the 2s orbital isn't full.
• B. 1s²2s²2p⁶3s¹3p¹: 3s has only 1 electron while another is in 3p. This is also an excited state, because the electron should be in 3s (the ground state would be 3s²).
• C. 1s²2s¹2p³: 2s isn't full while 2p is occupied. This is an excited state. The ground state would be 1s²2s²2p² (carbon).
• D. 1s²2s²2p⁶3s⁰: This is 1s²2s²2p⁶, which is neon, with every subshell filled in the correct order. It is a ground state (the 3s⁰ just means that subshell is empty).

Conclusion: Only option D has electrons filled in the proper order with no electron promoted to a higher level.

Tip: To spot a ground state, check for a "gap": if a lower subshell isn't full while a higher one has electrons, it is an excited state. No gaps means ground state.