(a) Transition metals show variable oxidation states because of small energy difference between (n-1)d and ns orbitals and as a result both (n-1)d and ns electrons take part in bond formation.
(b) The electrode potential of metals is decided by the following factors:
1. Enthalpy of atomisation
2. Ionisation enthalpy
3. Hydration enthalpy
The unique behaviour of Cu, having a positive E° is due to its high energy of atomization and low hydration enthalpy. The high energy to transform Cu(s) to Cu2+ (aq) is not balanced by its hydration enthalpy. Therefore, Evalue for (Cu2+/Cu) is positive.
(c) Higher oxidation state of Mn with fluorine is +4 whereas with oxygen is +7. Both O and F due to their small size and highest electronegativity can oxidize the transition metal to their highest oxidation state as it can easily unpair the electrons of metal. Among O and F, O is more superior to stabilize the highest oxidation state (+7 for Mn) of metal due to their ability to form multiple bonds.
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