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

Options

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

Answer

D

Explanation

In `._(23)V=1s^(2), 2s^(2)2p^(6),3s^(2)3p^(6)3d^(3),4s^(2)`
Third electron which is removed to give third ionisation potential, belongs to `3d^(3)` subshell.
`._(24)Cr=1s^(2),2s^(2),2p^(6),3s^(2)3p^(6),4s^(1)`
Third electron which is removed to give 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 to give 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 of d-subshell.
The `3d^(5)` -subshell has highest stability in all because it is half-filled subshell. So, Mn shows highest third ionsation potential.