1. Why there is an abnormally low value of the 3rd ionization enthalpy of lanthanium,gadolinium and lutetium observed?

This is because of extra stability of half-filled and completely filled f subshells.
The valence shell configurations are:

La: 4f0 5d1 6s2. Thus, La3+: 4f(f subshell empty) - stable
Gd: 4f​7 5d​1 6s​2. Thus, Gd3+: 4f​7 (f subshell half-filled) - stable
Lu: 4f14 5d1 6s2. Thus, Lu3+: 4f14 (f subshell completely filled) - stable

i.e. the third electron will be removed from 5d subshell. After its removal, the three elements will attain the above configurations which are quite stable. This is because half-filled and completely filled subshells impart extra stability to the configuration of any element. So, it would be easier to remove the third electron. Hence, third ionization energy will be lower than the first and second (second being higher than the first).



 

  • 6
What are you looking for?