What is the electron configuration for a neutral calcium atom?

• A. 1s^2 2s^2 2p^6 3s^2 3p^6 3d^{10} 4s^2
• B. 1s^2 2s^2 2p^6 3s^2 3p^6 4p^2
• C. 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2
• D. 1s^2 2s^2 2p^6 3s^2 3p^6 3d^2

Answer: (C)

Verified step by step guidance

1. Identify the atomic number of calcium, which tells you the number of electrons in a neutral atom. Calcium has an atomic number of 20, so it has 20 electrons.
2. Recall the order in which electron orbitals are filled according to the Aufbau principle: 1s, 2s, 2p, 3s, 3p, 4s, then 3d, and so on.
3. Distribute the 20 electrons into the orbitals following this order, making sure to fill each orbital to its maximum capacity before moving to the next: 1s (2 electrons), 2s (2 electrons), 2p (6 electrons), 3s (2 electrons), 3p (6 electrons), and then 4s (2 electrons).
4. Note that the 3d orbital is not filled before the 4s orbital for calcium because 4s has lower energy and fills first in this case.
5. Write the full electron configuration by combining all the filled orbitals with their respective electron counts: \$1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 4s^{2}$.