In neutral iron, how many unpaired electrons are in the 3d subshell?

• A. 2 unpaired
• B. 4 unpaired
• C. 0 unpaired
• D. 6 unpaired

Answer: (B)

Hund's rule and how electrons fill a d subshell are key. The 3d subshell has five degenerate orbitals. With six electrons in neutral iron, the first five electrons spread out to occupy each of the five d orbitals singly, giving five unpaired electrons. The sixth electron then pairs up with one of those, so one orbital becomes paired and the number of unpaired electrons drops to four. So, in neutral iron, the 3d subshell contains four unpaired electrons (consistent with the configuration [Ar] 3d6 4s2). This arrangement also helps explain iron’s magnetic behavior.