Unit 8 The D And F Block Elements (Intext Questions-4)
Intext Question
8.5 How would you account for the irregular variation of ionisation enthalpies (first and second) in the first series of the transition elements?
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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 $d^{0}, d^{5}, d^{10}$. Since these states are exceptionally stable, their ionization enthalpies are very high.
In case of first ionization energy, $\mathrm{Cr}$ has low ionization energy. This is because after losing one electron, it attains the stable configuration $\left(3 d^{5}\right)$. On the other hand, $\mathrm{Zn}$ has exceptionally high first ionization energy as an electron has to be removed from stable and fully-filled orbitals $\left(3 d^{10} 4 s^{2}\right)$.
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 $\mathrm{Cr}$ and $\mathrm{Cu}$ have exceptionally high second ionization energies as after losing the first electron, they have attained the stable configuration $\left(\mathrm{Cr}^{+}: 3 d^{5}\right.$ and $\mathrm{Cu}^{+}: 3 d^{10}$ ). Hence, taking out one electron more from this stable configuration will require a lot of energy.
