Question

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 ionisation enthalpy?

• A. Vanadium (Z = 23)
• B. Chromium (Z = 24)
• C. Iron (Z = 26)
• D. Manganese (Z = 25)

Answer 1

Answer: (D)

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