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 ionization enthalpy?

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

Answer 1

Answer: (D)

In ${}_{23}V=1s^2,2s^{2}2p^6,3s^{2}3p^{6}3d^3,4s^2$ third electron which is removed in third ionization enthalpy belongs to $3d^3$ sub-shell. ${}_{24}Cr=1s^2,2s^2,2p^6,3s^{2}3p^{6}3d^5,4s^1$ Third electron which is removed in third ionisation potential belongs to $3d^4$ sub-shell. ${}_{26}Fe=1s^2,2s^{2}2p^6,3s^{2}3p^{6}3d^6,4s^2$ Third electron which is removed in third ionisation enthalpy belongs to $3d^6$ sub-shell. ${}_{25}Mn=1s^2,2s^{2}2p^6,3s^{2}3p^6,3d^5,4s^2$ Third electron which is removed in third ionisation potential belongs to $3d^5$ 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 $3d^5$ 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.