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)

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