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Question
How would you account for the irregular variation of ionization enthalpies (first and second) in the first series of the transition elements?
Solution 1
Ionization enthalpies are found to increase in the given series due to a continuous filling of the inner d-orbitals. The irregular variations of ionization enthalpies can be attributed to the extra stability of configurations such as d0, d5 and d10. Since these states are exceptionally stable, their ionization enthalpies are very high.
In terms of first ionization energy, Cr has low ionization energy. This is because after losing one electron, it attains the stable configuration (3d5). On the other hand, Zn has exceptionally high first ionization energy, as an electron has to be removed from stable and fully-filled orbitals (3d10 4s2).
The second ionization energies are higher than the first since it becomes difficult to remove an electron when an electron has already been removed. Also, elements like Cr and Cu have exceptionally high second ionization energies, as after losing the first electron, they have attained the stable configuration (Cr+: 3d5 and Cu+: 3d10). Hence, taking out one electron more from this stable configuration will require a lot of energy.
Solution 2
The irregular variations in ionization enthalpy are due to differences in the stability of different 3d configurations (e.g., d0, d5, d10 are unusually stable).
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