What is the ground-state electron configuration for Fe2+ in noble-gas shorthand?

• A. [Ar] 3d6
• B. [Ar] 3d5
• C. [Ar] 3d7
• D. [Ar] 3d6 4s0

Answer: (A)

When removing electrons to form Fe2+, you take them from the outermost shells first. Iron in its neutral state is [Ar] 3d6 4s2. The two electrons are removed from the 4s orbital before touching the 3d electrons, because the 4s electrons are higher in energy once you start ionizing. That leaves [Ar] 3d6 4s0. In noble-gas shorthand, empty s orbitals are usually omitted, so the standard way to write the ground-state Fe2+ configuration is [Ar] 3d6. Some contexts might also write [Ar] 3d6 4s0, but the concise and widely accepted form is [Ar] 3d6.