JEE Main · 2010 · 11 Apr 2010 · Offline · d and f Block Elements

The standard potentials $E_{{\mathrm{M}}^{2+}/M}^{\circ }$ for these metals are all negative, and we are asked to order their magnitudes. The measured values are about $-1.18$ V for Mn, $-0.91$ V for Cr, $-0.44$ V for Fe and $-0.28$ V for Co. Manganese has the most negative value, helped by the stability of the half-filled ${\mathrm{d}}^5$ configuration of $\mathrm{M}{\mathrm{n}}^{2+}$, followed by chromium, then iron, with cobalt the least negative. The order of the negative values is $\mathrm{Mn}>\mathrm{Cr}>\mathrm{Fe}>\mathrm{Co}$.