(i) Name the element of 3d transition series which shows maximum number of oxidation
states. Why does it show so ?
(ii)Which transition metal of 3d series ha positive `E^(@)(M^(2+)//M)` value and why ?
(iii) Out of `Cr^(3+) and Mn^(3+)`, which is a stronger oxidizing agent and why ?
(iv) Name a member of the lanthanoid series which is well known to exhibit + 2 oxidation state.
(v) Complete the following equation : `MnO_(4)^(-) + 8H^(+) + 5e^(-) to ` .

Explanation

In 3d -series of transition metals, manganese has an atomic number of 25 that gives tha
electronic configuration as `[Ar] 3d 4s2,` where we see that the maximum number of ltbr. Unpaired electrons is found in manganese atom, so, it can show a maximum oxidation state upto + 7.
(ii) Copper is the transition metal of 3d series that exhibits positive `MnO_(4)^(-) + 8H^(+) + 5e^(-) to ` value of a metal
depends on the energy changes involved in the following :
1. Sublimation energy : The energy required for converting one mole of an atom from
Solid state to the gaseous state.
`M_(g) to underset("Sublimation energy")(M_((g)) Delta s H = high`
2. Ionisation energy : The energy required to take out electons from one mole of atom
in the isolated gaseous state. `M_((g)) to M_((g)) DeltaH = high` ("Ionisation energy")
3. Hydration energy : The energy released when one mole of ions are hydrated .
`M_((g)) to M_((g)) Delta s H ("low energy") ("Hydration energy") `
Since, copper has a high energy of atomisation and low hydration energy, the `E(@) (M^(2+)//M)` value for copper is positive.
(iii) Out of `Cr^(3+) and Mn^(3+), Mn^(3+)` is a stronger oxidising agent because it has 4 electrons in its
valence shell and when it gains one electron to form `Mn^(2+)`, it results in the half-filled `(d^(5))`
configuration that has extra stabillity.
(iv) Europium (Eu) is well-known to echibit + 2 oxidation state due to its half filled forbital in + 2 oxidation state.
(v) `MnO_(4)^(-)+8H^(+)+ 5e to Mn^(2+) + 4H_(2)O`