In the abbreviated electron configuration for iron, which noble gas core is used?

• A. [Ne]
• B. [Kr]
• C. [Xe]
• D. [Ar]

Answer: (D)

Representing inner electrons with a noble gas core means you stop the core once you reach the last fully filled shell before the valence electrons you’re listing. For iron (26 electrons), the electrons fill up to the 3p6 level, which corresponds to Argon with 18 electrons. That leaves 8 electrons to place in higher shells, and those occupy 4s and 3d as 4s2 3d6. So the abbreviated configuration is [Ar] 4s2 3d6. The other noble gases would imply a different, larger or smaller inner core that doesn’t match iron’s actual inner-shell filling.