JEE Main · 2020 · 08 Jan 2020 · Shift 1 · Classification of Elements and Periodicity

The third ionization enthalpy removes an electron from the doubly charged ion. For iron, $[\mathrm{Ar}]3d^{6}4s^2$, the first two electrons leave the $4s$ level to give $3d^6$, and removing the third takes $3d^6$ to the half-filled, extra-stable $3d^5$ configuration. Reaching that stable half-filled state makes the third electron comparatively easy to pull off, so among manganese, iron, cobalt and nickel it is iron that has the lowest third ionization enthalpy.