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The following sentence is stated in my current chemistry book:

The first ionization enthalpy of zinc is higher because it represents an ionization from the $\rm 4s$ level.

Now zinc does have an ionization enthalpy of $\pu{906 kJ/mol}$, which is higher than copper's $\pu{745 kJ/mol}$ or nickel's $\pu{736 kJ/mol}$ in the same series. But I am not sure that the reason for this increase stated in the book is correct. For example, even copper has an ionization from the $\rm 4s$ level, but it does not have anywhere near as high ionization enthalpy as zinc. Is the book correct, or is there some other reason for this?

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    $\begingroup$ The book is incorrect. Cu ionisation is also from 4s. The reason is purely because of the increase in effective nuclear charge in Zn. $\endgroup$ – orthocresol May 14 '16 at 16:21
  • $\begingroup$ Do you know the real reason behind this unexpected increase $\endgroup$ – Abhishek Mhatre May 15 '16 at 15:13
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It is because the shell is complete.

Zinc's electronic configuration is $\rm [Ar] 3d^{10} 4s^2$, which is a complete configuration for a transition metal. The next element in the same period is $\ce{Ga}$, which starts filling $\rm 4p$ orbitals ($\rm [Ar] 3d^{10} 4s^2 4p^1$).

The zinc's electronic configuration causes a sort of special stability because of the filled electronic states. Also, the effective nuclear charge is greater than $\ce{Cu}$'s, so it's more difficult to ionize $\ce{Zn}$.

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