Question

How would you account for the irregular variation of ionization enthalpies (first and second) in the first series of the transition elements?

Answer 1

Hint: The ionization enthalpies of the transition elements are always higher than the s-block elements but are always smaller than the p-block elements. In the transition series, there are some exceptional configurations that lead to the extra stability of the elements.

Complete step by step answer:
We know that the ionization enthalpy or ionization energy of an element is the energy required to extract an electron from its outermost shell and convert it into an ion.
The first and second ionization enthalpies of the transition elements lie between the s-block and p-block elements, or we can say that the ionization energy is always higher than the s-block elements but are lower than that of p-block elements.
The ionization enthalpies of the first transition series increase as we move along the period but the series have some irregularities.
The fact that the increase in nuclear charge with an increase in atomic number as we move along the period reduces the size and increases the ionization enthalpy. The irregular trend in the ionization energies of the first series is due to the fact that the removal of electrons alters the relative energy of $4s$ and $3d$ orbitals. So, reorganization energy accompanies the ionization. This results in the release of exchange energy which increases as the number of electrons increases in the ${{d}^{n}}$ configuration and also from the transfer of s-electrons into the d-orbital. So the Cr has the first ionization very low because the removal of one electron stable configuration (${{d}^{5}}$ ) is attained. Zinc has the highest ionization enthalpy because it has both the orbitals fully filled i.e., $3{{d}^{10}}4{{s}^{2}}$.

Note: It is due to the fact that higher ionization enthalpies of copper, nickel, and zinc show a maximum oxidation state of +2. All the series in the transition metals (second and third) have irregular ionization enthalpy due to electronic configuration.