1. Tardigrade
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  4. Four successive members of the first row transition elements are listed below with their atomic numbers. Which one of them is expected to have the highest third ionization enthalpy?

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Q. Four successive members of the first row transition elements are listed below with their atomic numbers. Which one of them is expected to have the highest third ionization enthalpy?

CMC MedicalCMC Medical 2012

Solution:

In $ _{23}V=1{{s}^{2}},2{{s}^{2}}2{{p}^{6}},3{{s}^{2}}3{{p}^{6}}3{{d}^{3}},4{{s}^{2}} $ third electron which is removed in third ionization enthalpy belongs to $ 3{{d}^{3}} $ sub-shell. $ _{24}Cr=1{{s}^{2}},2{{s}^{2}},2{{p}^{6}},3{{s}^{2}}3{{p}^{6}}3{{d}^{5}},4{{s}^{1}} $ Third electron which is removed in third ionisation potential belongs to $ 3{{d}^{4}} $ sub-shell. $ _{26}Fe=1{{s}^{2}},2{{s}^{2}}2{{p}^{6}},3{{s}^{2}}3{{p}^{6}}3{{d}^{6}},4{{s}^{2}} $ Third electron which is removed in third ionisation enthalpy belongs to $ 3{{d}^{6}} $ sub-shell. $ _{25}Mn=1{{s}^{2}},2{{s}^{2}}2{{p}^{6}},3{{s}^{2}}3{{p}^{6}},3{{d}^{5}},4{{s}^{2}} $ Third electron which is removed in third ionisation potential belongs to $ 3{{d}^{5}} $ sub-shell. In all elements shell and sub-shells are same. Required amount of energy (enthalpy) is based upon the stability of d-subshell. The $ 3{{d}^{5}} $ sub-shell has highest stability in all because it is half-filled sub-shell (while other are incomplete). So, Mn shows highest third ionisation potential.