What is the complete electron configuration for calcium (Ca)?

• A. 1s2 2s2 2p6 3s2 3p4 4s2
• B. ${}^{\mathrm{}}$1s2 2s2 2p6 3s2 3p6 4s2
• C. 1s2 2s2 2p6 3s2 3p6 3d2
• D. 1s2 2s2 2p6 3s2 3p6 4s1

Answer: (B)

Verified step by step guidance

1. Identify the atomic number of calcium (Ca), which tells you the total number of electrons to place in the electron configuration. 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. Start filling the orbitals with electrons, beginning with the lowest energy level: fill 1s with 2 electrons, then 2s with 2 electrons, 2p with 6 electrons, 3s with 2 electrons, and 3p with 6 electrons. This accounts for 18 electrons so far.
4. Next, place the remaining 2 electrons in the 4s orbital, since it has lower energy than the 3d orbital for calcium.
5. Write the complete electron configuration by combining all filled orbitals: \$1s^2 2s^2 2p^6 3s^2 3p^6 4s^2$.