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Question
Compare the stability of +2 oxidation state for the elements of the first transition series.
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Solution 1
| Sc | +3 | ||||||
| Ti | +2 | +3 | +4 | ||||
| V | +2 | +3 | +4 | +5 | |||
| Cr | +2 | +3 | +4 | +5 | +6 | ||
| Mn | +2 | +3 | +4 | +5 | +6 | +7 | |
| Fe | +2 | +3 | +4 | +6 | |||
| Co | +2 | +3 | +4 | ||||
| Ni | +2 | +3 | +4 | ||||
| Cu | +1 | +2 | |||||
| Zn | +2 |
Solution 2
Firstly, in the first half of the transition series, the sum of the first- and second-ionisation enthalpies increases with atomic number. Hence, the standard reducing potential (E°) is low and negative. Hence, the tendency to form M2+ ions decreases. Hence, the +2 oxidation state is more stable in the first half. The greater stability of the +2 oxidation state is due to half-filled d-subshells (d5) in Mn2+, fully filled d-subshells (d10) in Zn2+, and high negative hydration enthalpy in nickel.
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