How would you account for the irregular variation of ionization enthalpies (first and second) in the first series of the transition elements?
The ionization energy increases due to the gradual filling of electrons in the d- subshells. The irregular variation of ionization energy is due to the fact that half-filled and completely filled subshells are more stable and have very high ionization energy.
In case of first ionization energy, Cr ([Ar]3d54s1) attains the stable configuration (3d5) by losing one electron from s-subshell and hence, it has low ionization energy.
Whereas, Zn has high ionization energy because it has completely filled subshells and are very stable.
Second ionization energies are higher than the first since it becomes difficult to remove an electron when an electron has to be removed from a stable structure, which was formed due to the removal of the first electron.