संक्रमण तत्त्वों की प्रथम श्रेणी में आयनन एन्थैल्पी (प्रथम और द्वितीय) में अनियमित परिवर्तन को आप कैसे समझायेंगे?

Ionization enthalpies are found to increase in the given series due to a continuous filling of the inner $d$ -orbitals. The irreqular 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, $Cr$ 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, $Z n$ 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 $Cr$ and $Cu$ have exceptionally high second ionization energies as after losing the first electron, they have attained the stable configuration $( Cr ^{+}: 3 d^{5} $ and $ Cu ^{+}: 3 d^{10})$. Hence, taking out one electron more from this stable configuration will require a lot of energy.