According to the aufbau principle, which orbital is filled first when building up the electron configuration of an atom?

• A. 4s
• B. 1s
• C. 3d
• D. 2p

Answer: (B)

Verified step by step guidance

1. Understand the aufbau principle: it states that electrons fill atomic orbitals starting from the lowest energy level to higher energy levels.
2. Recall the order of orbital filling based on increasing energy: 1s, 2s, 2p, 3s, 3p, 4s, 3d, and so on.
3. Identify the lowest energy orbital available in an atom, which is the 1s orbital.
4. Recognize that according to the aufbau principle, the 1s orbital is filled first before any other orbitals such as 4s, 3d, or 2p.
5. Therefore, the first orbital to be filled when building up the electron configuration of an atom is the 1s orbital.