What is the electron configuration for the neutral iron atom (Fe, Z=26) in full notation?

• A. 1s2 2s2 2p6 3s2 3p6 4s2 3d6
• B. 1s2 2s2 2p6 3s2 3p6 3d6 4s2
• C. [Ar] 3d6 4s2
• D. 1s2 2s2 2p6 3s2 3p6 4s1 3d7

Answer: (A)

Electrons fill orbitals in order of increasing energy, so we build up Fe’s 26 electrons by going through the 1s, 2s, 2p, 3s, 3p levels first, then the 4s sublevel before the 3d sublevels in the typical filling sequence. After counting the inner shells, the next two electrons go into the 4s orbital and the remaining six go into the 3d orbitals, giving a full notation expansion of 1s2 2s2 2p6 3s2 3p6 4s2 3d6. This explicit full notation shows all electrons laid out in their order of filling, which is what the question asks for. Some alternatives shorten the notation or present the d and s electrons in a different order, which either omits electrons or implies a different convention, so they aren’t the proper full representation for neutral iron. Shortened forms like [Ar] 3d6 4s2 are equivalent in content but not in full notation, and configurations with 4s1 3d7 would describe a different or excited state.