A) Vanadium (Z = 23)
B) Chromium (Z = 24)
C) Iron (2 = 26)
D) Manganese (Z = 25)
Correct Answer: D
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 ionisation potential 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 m third ionisation potential enthalpy belongs to \[3{{d}^{5}}\] 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 potential 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 or enthalpy.
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