Chapter 4 · Chemistry
One Element, Six Oxidation States: Why Transition Metals Vary
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The ns and (n−1)d electrons of a transition metal are close in energy, so both can take part in bonding. Manganese (3d⁵ 4s²) shows every state from +2 to +7; the most states occur in the middle of the series, Sc shows only +3 and Zn only +2, and the states change in steps of one.
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Zinc Fizzes, Copper Stays Silent: Electrode Potentials of the 3d MetalsCopper is the only 3d metal with a positive E°(M²⁺/M) (+0.34 V), so it gives no H₂ with dilute acids. Turning Cu(s) into Cu²⁺(aq) costs more (atomisation + two ionisations) than hydration pays back: 921 against zinc's 711 kJ/mol. Stable d⁵ and d¹⁰ bend the trend: Mn and Zn dip, Mn³⁺ is a strong oxidant, Fe³⁺ is stable.Chemistry
Key points
- ns and (n−1)d electrons are close in energy, so both take part in bonding: variable oxidation states.
- Mn (3d⁵ 4s²) shows +2 to +7: Mn²⁺ (+2), MnO₂ (+4), KMnO₄ (+7).
- The most states occur in the middle of the series; Sc only +3, Zn only +2.
- States differ by one (Fe²⁺/Fe³⁺); in the p-block usually by two (Sn²⁺/Sn⁴⁺).
Oxidation states across the 3d series
| Sc | Ti | V | Cr | Mn | Fe | Co | Ni | Cu | Zn |
|---|---|---|---|---|---|---|---|---|---|
| +3 | +2 to +4 | +2 to +5 | +2 to +6 | +2 to +7 | +2, +3, +4, +6 | +2 to +4 | +2 to +4 | +1, +2 | +2 |
Up to manganese, the highest oxidation state equals the number of 4s + 3d electrons: Sc +3, Ti +4, V +5, Cr +6, Mn +7.
Steps of one, not two
Transition-metal states change one unit at a time (V +2, +3, +4, +5; Fe²⁺/Fe³⁺; Cu⁺/Cu²⁺). In the p-block the common states usually differ by two (Sn²⁺/Sn⁴⁺, Pb²⁺/Pb⁴⁺).
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