Nature of Oxides of 3d Series
Trends in Acidic, Basic, and Amphoteric character with Oxidation State.
By chemca Team • Updated Jan 2026
The transition metals of the 3d series form oxides with general formulas MO, $M_2O_3$, $MO_2$, $M_2O_5$, $MO_3$, etc. The nature of these oxides changes drastically from Basic to Amphoteric to Acidic as the oxidation state increases.
1. The General Trend
Oxidation State vs. Acidity
As the oxidation number of the metal increases, the ionic character decreases (Fajan's Rule) and covalent character increases, making the oxide more acidic.
| Oxidation State | Nature | Bonding | Examples |
|---|---|---|---|
| Low (+2, +3) | Basic | Ionic | $MnO, FeO, NiO$ |
| Intermediate (+3, +4) | Amphoteric | Intermediate | $Cr_2O_3, MnO_2, V_2O_4$ |
| High (+5, +6, +7) | Acidic | Covalent | $V_2O_5, CrO_3, Mn_2O_7$ |
2. Element-wise Analysis
Manganese Oxides (Mn)
Manganese exhibits the widest range of oxidation states, clearly showing the trend.
$$ \underset{\text{Basic}}{MnO (+2)} < \underset{\text{Basic}}{Mn_2O_3 (+3)} < \underset{\text{Amphoteric}}{MnO_2 (+4)} < \underset{\text{Acidic}}{Mn_2O_7 (+7)} $$
Evidence:
- $MnO$ dissolves in acids forming $Mn^{2+}$ salts (Basic).
- $Mn_2O_7$ is an oily liquid (covalent) that dissolves in water to form Permanganic Acid ($HMnO_4$).
Chromium Oxides (Cr)
$$ \underset{\text{Basic}}{CrO (+2)} < \underset{\text{Amphoteric}}{Cr_2O_3 (+3)} < \underset{\text{Acidic}}{CrO_3 (+6)} $$
Evidence:
- $Cr_2O_3$ dissolves in acids ($Cr^{3+}$) and alkalis ($CrO_2^-$), proving amphoteric nature.
- $CrO_3$ (Chromic anhydride) dissolves in water to form Chromic Acid ($H_2CrO_4$).
Vanadium Oxides (V)
$$ \underset{\text{Basic}}{VO (+2)} < \underset{\text{Basic}}{V_2O_3 (+3)} < \underset{\text{Amphoteric}}{VO_2 (+4)} < \underset{\text{Amphoteric/Acidic}}{V_2O_5 (+5)} $$
Note on $V_2O_5$: While predominantly acidic (reacts with alkalis to form Vanadates $VO_4^{3-}$), it is technically amphoteric as it also reacts with acids to give $VO_2^+$ salts.
3. Proof of Nature (Chemical Reactions)
1. Basic Nature ($MnO$)
Reacts with acid to form salt and water.
$$ MnO + 2HCl \rightarrow MnCl_2 + H_2O $$
2. Amphoteric Nature ($ZnO, Cr_2O_3$)
Reacts with both acids and bases.
Acid: $$ ZnO + 2HCl \rightarrow ZnCl_2 + H_2O $$
Base: $$ ZnO + 2NaOH \rightarrow \underset{\text{Sodium Zincate}}{Na_2ZnO_2} + H_2O $$
3. Acidic Nature ($Mn_2O_7, CrO_3$)
Reacts with water to form acids.
$$ 2CrO_3 + H_2O \rightarrow H_2Cr_2O_7 \text{ (Dichromic Acid)} $$
$$ Mn_2O_7 + H_2O \rightarrow 2HMnO_4 \text{ (Permanganic Acid)} $$
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