The electron configuration of $_{26}Fe^{3+}$ is?

Analyze the question: You need the electron configuration of the Fe³⁺ ion, not the neutral Fe atom.

Concept:

1. Write the configuration of the neutral atom first.
2. For transition metals, when forming cations, electrons are removed from the 4s orbital first (the outermost shell), and then from 3d.

Solution:

• Neutral Fe (Z = 26): 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶, which is [Ar] 4s² 3d⁶
• To form Fe³⁺, remove 3 electrons: • Remove 2 electrons from 4s, leaving 4s⁰ • Remove 1 electron from 3d, so 3d⁶ becomes 3d⁵
• Fe³⁺: [Ar] 4s⁰ 3d⁵

Check: Total electrons = 18 (Ar) + 5 = 23, which equals 26 − 3. ✓

Checking the options:

• A: [Ar]4s²3d⁶ is neutral Fe, not the ion.
• B: [Ar]4s²3d³ has 23 electrons but the wrong arrangement, because 4s is lost before 3d.
• C: [Ar]4s¹3d⁴ has 23 electrons but is not the correct arrangement. It is also a stable half-filled d subshell.

Tip: For transition metal cations, remember "4s leaves first." Fill 4s before 3d when building the atom, but remove from 4s before 3d when making ions. Fe³⁺ has a half-filled 3d⁵ configuration, which explains its extra stability.