The atomic number of iron is 56. The number of unpaired electrons in the Fe3+ ion is

Note on the question: Iron's atomic number is actually 26, not 56 (56 is its mass number). The question has a typo, so we use Z = 26, which is what the question intends.

Step 1: Electronic configuration of Fe (Z = 26) 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶ Or in short form: [Ar] 4s² 3d⁶

Step 2: Form Fe³⁺ When a transition metal forms a cation, it loses the 4s electrons first, then 3d electrons.

• Remove 2 electrons from 4s, giving 4s⁰
• Remove 1 electron from 3d, giving 3d⁵

So Fe³⁺ = [Ar] 3d⁵

Step 3: Distribute the 3d⁵ electrons (Hund's rule) Each of the five 3d orbitals gets one electron before any pairing:

↑ | ↑ | ↑ | ↑ | ↑

All 5 electrons are unpaired.

Answer: E (5 unpaired electrons)

Tip: Fe³⁺ has a half-filled d subshell (3d⁵), which is especially stable. For any transition metal ion, remember "4s leaves first." Also, Fe²⁺ has 3d⁶ (4 unpaired), while Fe³⁺ has 3d⁵ (5 unpaired). This comparison is a favorite exam question.