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The d-and f-Block Elements — Chemistry STD 12 Science — Question

Rajasthan BoardEnglish MediumSTD 12 ScienceChemistryThe d-and f-Block Elements3 Marks
Question
Explain the variation in ionization enthalpies of transition element in $3d$ series.
✓

Answer

$\rightarrow$ Due to an increase in nuclear charge which accompanies the filling of the inner d orbitals, there is an increase in ionization enthalpy along each series of the transition elements from left to right.
$\rightarrow$ The irregular trend in the first ionization enthalpy of the $3d$ metals, though of little chemical significance, can be accounted for by considering that the removal of one electron alters the relative energies of $4s$ and $3d$ orbitals.
$\rightarrow$ So the unipositive ions have $d^n$ configurations with no $4s$ electrons. There is thus, a reorganization energy accompanying ionization with some gains in exchange energy as the number of electrons increases and from the transference of s electrons into d orbitals.
$\rightarrow$ There is the generally expected increasing trend in the values as the effective nuclear charge increases. However, the value of $Cr$ is lower because of the absence of any change in the d configuration and the value for $Zn$ higher because it represent an ionization from the $4s$ level.
$\rightarrow$ The lowest common oxidation state of these metals is $+2.$ To form the $M^{2+}$ ions fron the gaseous atoms, the sum of the first and second ionization energies is required in addition to the enthalpy of atomization for each element.
$\rightarrow$ The dominant term is the second ionization enthalpy which shows unusually high values for $Cr$ and $Cu$ where the $d^5$ and $d^{10}$ configurations of the $M^+$ ions are disrupted, with considerable loss of exchange energy. The value for $Zn$ is correspondingly low as the ionization consists of the removal of an electron which allows the production of the stable $d^{10}$ configuration.
$\rightarrow$ The trend in the third ionization enthalpies is not complicated by the $4s$ orbital factor and shows the greater difficulty of removing and electron from the $d^5 (Mn^{2+})$ and $d^{10} (Zn^{2+})$ ions superimposed upon the general increasing trend. In general, the third ionization enthalpies are quite high and there is a marked break between the values for $Mn^{2+} $ and $Fe^{2+}.$
$\rightarrow$ Also, the high values for copper, nickel and zinc indicate why it is difficult to obtain oxidation state greater than two for these elements.

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