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Q.
Which one of the following first row transition elements is expected to have the highest third ionization enthalpy?
The d-and f-Block Elements
Solution:
$M^{2+} \xrightarrow[-e^-]{IE_3} M^{3+} $
$Mn^{2+}$ has the most stable configuration, i.e., $3d^5$ (all the five d-orbitals are singly occupied). Hence, to remove electron from $Mn^{2+}$ ion requires more energy, i.e., the third ionization enthalpy of manganese is highest.