---
title: "Stability of Fe³⁺ vs Fe²⁺"
subject: "Engineering Chemistry"
chapter: 5
key: "semester_2/engineering_chemistry:fe3-stable"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/fe3-stable
site: BCT Engineering
---

# Stability of Fe³⁺ vs Fe²⁺

Engineering Chemistry · Chapter 5: 3-d Transition Elements and their Applications

- Asked in: 2075 Baisakh, 1 marks
- Video: [Which one is more stable between Fe2+ and Fe3+? | which one is more stable fe+2 or fe+3 and why](https://www.youtube.com/watch?v=4MrAlviRow0) — Active Classroom
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/fe3-stable
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-5

## Question

Explain why Fe³⁺ compounds are more stable than Fe²⁺.

## Short answer

Fe³⁺ (3d⁵) has a stable half-filled d subshell with high exchange energy. Fe²⁺ (3d⁶) readily loses one electron to attain this d⁵ configuration, so Fe³⁺ compounds are more stable and Fe²⁺ is easily oxidised.

## Answer

Fe³⁺ has the configuration 3d⁵, a half-filled d subshell, which has extra stability due to symmetrical electron distribution and high exchange energy. Fe²⁺ is 3d⁶, and loses one electron easily to reach the stable d⁵ state (Fe²⁺ → Fe³⁺ + e⁻). So Fe²⁺ compounds are readily oxidised by air, whereas Fe³⁺ compounds are more stable.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Why is Fe3+ more stable than Fe2+?
   A. Fe3+ is 3d10
   B. Fe3+ is 3d6
   C. Fe3+ is 3d5, a half-filled subshell with extra stability
   D. Fe3+ has no electrons
   Answer: C. Half-filled d5 has symmetry and exchange energy. (Key point: Config)

2. Fe2+ is
   A. 3d5
   B. 3d10
   C. 3d4
   D. 3d6
   Answer: D. It loses one electron to become d5. (Key point: Fe2+ config)

3. What happens to Fe2+ compounds in air?
   A. They are readily oxidised to Fe3+
   B. They are reduced to Fe
   C. They remain unchanged
   D. They turn to gas
   Answer: A. Fe2+ loses an electron to reach d5. (Key point: Air)

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