Stability Of Oxidation States

The special stability of certain oxidation states among transition metals often reflects the extra stability of half-filled or fully filled d-subshells. Manganese in the +2 state, Mn^2+, has the electronic configuration 3d^5, an exactly half-filled d-subshell. This arrangement is especially stable because of its symmetrical distribution of electrons and the favourable exchange energy associated with five parallel-spin electrons. As a result, Mn^2+ is more stable and less easily oxidised than the +2 states of its neighbours, and manganese resists further oxidation in this state. The option of a fully filled d-orbital is wrong, since 3d^5 is half-filled, not full. The option of no d-electrons is incorrect, as Mn^2+ has five. The option that it is the largest ion is irrelevant to the electronic stability. This half-filled stability is a key JEE Advanced d-block concept. Working through the logic step by step, rather than memorising the result, makes it clear why manganese governs the behaviour seen here. Carefully relating the data to the governing principle ensures the reasoning remains valid even when the numbers or species in the question are changed. Plausibility check: the parallel extra stability of the half-filled f-subshell in Eu^2+ and Gd^3+ confirms that half-filled subshells confer enhanced stability, supporting the reasoning for Mn^2+.