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The d block elements display several exceptional behaviours, and one of them is the irregular trend in ionization enthalpy of 3d elements.

The reason given by my textbook is very confusing to me so I'm unable to grasp it. I would appreciate it if someone could explain this statement to me in a simple way.

The irregular trend in the first ionisation enthalpy of the $\ce{3d}$ metals, can be accounted for by considering that the removal of one electron alters the relative energies of $\ce{4s}$ and $\ce{3d}$ orbitals. So the unipositive ions have $\ce{d^n}$ configurations with no $\ce{4s}$ electrons. There is thus, a reorganisation energy accompanying ionisation with some gains in exchange energy as the number of electrons increases and from the transference of s electrons into d orbitals

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Text source: NCERT (d & f block elements/ page 215)
Image source: Inorganic Chemistry Cengage Part-2 ( page 6.13)

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The trend in I.E of transitional elements are irregular for mainly two reasons:-

  1. Irregular trend of atomic radius
  2. Reorganization Energy

You might be knowing that Ionization Enthalpy is inversely proportional to Atomic Radius, so irregularity in trend of atomic radius also reflect in the trend of Ionization Enthalpy.

But the relation between atomic radius and ionization enthalpy isn't the sole reason for the trend. There comes a thing called Reorganization Energy. D block loses one electron from 4s orbital when the required energy is supplied which you may say it as ionization enthalpy but wait. There's another electron in the 4s orbital. That electron takes extra energy and jumps into d orbital. This is known as Reorganization Energy. Due to reorganization, the element attains a certain level of stability. Due to achievement of stability the element releases a energy known as Exchange Energy.

So the overall energy involvement you see is actually the (Initial Energy to remove electron)+(Reorganization Energy)+(Exchange Energy) which is equal to Ionization Energy.

Thus Ionization Enthalpy is affected by a variety of factors and so its trend is also irregular.

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