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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)

Melanie Shebel
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Ritwik Das
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  • Rekated: https://physics.stackexchange.com/q/240500/81224 – Apoorv Potnis Mar 04 '18 at 07:15
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    Your question could benefit if you explain to us how you interpreted the statement by the book, so we could tell you where you went wrong or missed anything. The link by Apoorv is great, and here are some more for you to read https://chemistry.stackexchange.com/questions/8357/why-does-the-3rd-electron-shell-start-filling-up-with-scandium https://chemistry.stackexchange.com/questions/151/why-do-elements-in-columns-6-and-11-assume-abnormal-electron-configurations https://chemistry.stackexchange.com/questions/18988/how-does-the-radial-distribution-function-of-vanadium-differ-from-that-of-calciu – Gaurang Tandon Mar 15 '18 at 14:27
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    The statement in your textbook is actually not true for all of the transition elements in the first row. Ti+ has a configuration of 3d2 4s1, contrary to the statement "unipositive ions have dn configuration with no 4s electrons". – Tan Yong Boon Mar 15 '18 at 14:46
  • read the 2nd answer in this link https://chemistry.stackexchange.com/questions/151/why-do-elements-in-columns-6-and-11-assume-abnormal-electron-configurations ..... esp. the chemguide link in it – vishesh jain Jul 14 '19 at 11:37

1 Answers1

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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.

ADITYA DAS
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