Which of the following electron configurations represents a phosphorus atom (atomic number 15) in an excited state?

• A. 1s^2 2s^2 2p^6 3s^2 3p^5
• B. 1s^2 2s^2 2p^6 3s^2 3p^2 4s^1
• C. 1s^2 2s^2 2p^6 3s^2 3p^3
• D. 1s^2 2s^2 2p^6 3s^1 3p^4

Answer: (D)

Verified step by step guidance

1. Recall that the atomic number of phosphorus is 15, which means a neutral phosphorus atom has 15 electrons.
2. Write the ground state electron configuration by filling orbitals in order of increasing energy: 1s, 2s, 2p, 3s, then 3p. For phosphorus, the ground state configuration is \$1s^2 2s^2 2p^6 3s^2 3p^3$.
3. Understand that an excited state occurs when one or more electrons are promoted to a higher energy orbital than in the ground state, without changing the total number of electrons.
4. Compare each given configuration to the ground state configuration to see if electrons have been moved to higher energy orbitals:
5. Identify the configuration where an electron from the 3s orbital is promoted to the 3p orbital, resulting in \$1s^2 2s^2 2p^6 3s^1 3p^4$, which represents an excited state of phosphorus.