How many electrons occupy the 4s subshell in neutral iron?

• A. 3
• B. 2
• C. 0
• D. 1

Answer: (B)

Two electrons occupy the 4s subshell in neutral iron. Iron has 26 electrons, and its ground-state configuration is [Ar] 3d6 4s2. The 4s subshell can hold up to two electrons, and in this element both slots are filled to give the most stable arrangement. The reason 4s electrons appear there before some 3d electrons is due to their relative energy levels during filling; after the arrangement is set, iron ends up with two electrons in 4s and six in 3d.