Question 8.5:How would you account for the irregular variation of ionization enthalpies (first and second) in the first series of the trans…

Answer:
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, d10. Since these states are exceptionally stable,
their ionization enthalpies are very high.
In case 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).
Second ionization
energies are higher than the first since it becomes difficult to
remove an electron when an electron has already been taken out. 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.