---
title: "Variable oxidation states of 3d elements"
subject: "Engineering Chemistry"
chapter: 5
key: "semester_2/engineering_chemistry:variable-oxidation-3d"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/variable-oxidation-3d
site: BCT Engineering
---

# Variable oxidation states of 3d elements

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

- Asked in: 2074 Bhadra, 1+2 marks; 2073 Magh, 2.5 marks; 2070 Magh, 2 marks; 2069 Bhadra; 2068 Bhadra, 2 marks; 2067 Chaitra, 2 marks (asked 6 times)
- Video: [Oxidation states of transition elements | The d-block elements | Chemistry | Khan Academy](https://www.youtube.com/watch?v=gPHJr2xijYs) — Khan Academy India - English
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/variable-oxidation-3d
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-5

## Question

Why do transition elements show variable oxidation states? Explain with reference to 3d series elements.

## Short answer

The (n-1)d and ns electrons have similar energies, so both ns and a variable number of (n-1)d electrons can participate in bonding. Successive ionisation energies rise gradually, allowing multiple stable oxidation states. In the 3d series, states typically differ by one unit, e.g., Fe²⁺ and Fe³⁺.

## Answer

Transition elements show variable oxidation states because the (n−1)d and ns electrons have very nearly the same energy, so both the ns electrons and a variable number of (n−1)d electrons can take part in bonding. Successive ionisation energies rise gradually (unlike the sudden jump after the s-electrons in s-block metals), so ions of several charges can form. In the 3d series the states usually differ by one unit (e.g. Fe²⁺, Fe³⁺), unlike p-block elements whose states differ by two (Sn²⁺, Sn⁴⁺).

#### Oxidation states of 3d elements
| Element | Sc | Ti | V | Cr | Mn | Fe | Co | Ni | Cu | Zn |
|---|---|---|---|---|---|---|---|---|---|---|
| Common oxidation states | +3 | +2, +3, +4 | +2, +3, +4, +5 | +2, +3, +6 | +2, +3, +4, +6, +7 | +2, +3 | +2, +3 | +2 | +1, +2 | +2 |

- Sc has one stable state (+3, loses 3 electrons to reach argon core) and Zn only +2 (3d¹⁰ stable), so they show little variability.
- The maximum state increases from Sc (+3) to Mn (+7), equal to the number of 3d + 4s electrons, then decreases after Mn because electrons are paired and the d orbitals become more stable.
- Examples: MnO₄⁻ (+7), Cr₂O₇²⁻ (+6), VO₂⁺ (+5).

**Stability:** the state with d⁰, d⁵ or d¹⁰ configuration is relatively stable (Sc³⁺, Mn²⁺, Fe³⁺, Zn²⁺). Higher states are stable in oxides and fluorides (high electronegativity), lower states in complexes with π-acceptor ligands such as CO.

## Check yourself

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

1. Why do transition elements show variable oxidation states?
   A. (n-1)d and ns electrons have nearly the same energy, so both can take part in bonding
   B. Only ns electrons are available
   C. They have no d electrons
   D. They have very high first ionisation energy only
   Answer: A. Close energy of ns and (n-1)d electrons lets a variable number be lost. (Key point: Cause)

2. How do 3d oxidation states typically differ?
   A. By two units
   B. By one unit (e.g. Fe2+, Fe3+), unlike p-block states differing by two
   C. By three units
   D. They do not differ
   Answer: B. p-block e.g. Sn2+, Sn4+. (Key point: Difference from p-block)

3. Which element shows the highest oxidation state in the 3d series?
   A. Sc (+3)
   B. Fe (+8)
   C. Mn (+7)
   D. Zn (+7)
   Answer: C. Maximum state rises from Sc (+3) to Mn (+7), then falls. (Key point: Maximum state)

4. Why do Sc and Zn show little variability?
   A. They have no electrons
   B. They lose all electrons
   C. They are non-metals
   D. Sc is stable only as +3; Zn only +2 (3d10 stable)
   Answer: D. Sc loses 3 electrons to reach an argon core; Zn keeps 3d10. (Key point: Sc and Zn)

## Related in Engineering Chemistry

- [Transition elements; oxidation states of 3d; alloys](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Atm-ox-alloy) — Engineering Chemistry, chapter 5
- [Variable oxidation states; industrial uses of 3d elements](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Avariable-ox-applications) — Engineering Chemistry, chapter 5
- [First-row configuration; variable oxidation states](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Aconfig-variable-oxidation) — Engineering Chemistry, chapter 5
- [Transition elements; non-typical 3d element](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Atm-not-typical) — Engineering Chemistry, chapter 5
- [Why 3d elements are transition; valency characteristics](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3A3d-transition-valency) — Engineering Chemistry, chapter 5
- [Transition elements; why so called; 3d configurations](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Atm-called-config) — Engineering Chemistry, chapter 5