Q.Which is more stable $Fe^{3+}$ or $Fe^{2+}$ ? Why ?

Scandium, the first 3d element, exhibits only one oxidation state, +3. Every other transition element shows variable oxidation states, because the (n-1)d and ns orbitals lie so close in energy that electrons from either (or both) can be lost. Across the 3d series, the number of accessible oxidation states rises then falls: it rises because more electrons become available through the first half of the series, and falls again as the number of paired d electrons increases toward the end. Scandium (first) has just one state (+3); manganese (middle) has the most, six states from +2 to +7; copper (last) is restricted to +1 and +2.

The relative stability among a metal's different oxidation states correlates with the extra stability of half-filled/fully-filled configurations -- Mn²⁺ (3d⁵) is markedly more stable than Mn⁴⁺ (3d³). For the heavier 4d/5d series, oxidation states range even more widely, from +3 (Y, La) up to +8 (Ru, Os) -- the highest oxidation state anywhere in the d-block -- typically only reached in compounds with strongly electronegative elements (RuO₄, OsO₄, WCl₆). Moving down a group, stability of the higher oxidation state increases while the lower state's stability decreases, the reverse of the p-block pattern. …