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

The irregular variation in ionisation enthalpies is due to the extra stability of the configuration like $d^{0}\ ,d^{5}\ ,d^{10}$ because these states are extremely stable and have high ionisation enthalpies.
In the case of chromium ($Cr$) has low 1st IE because after losing one electron it attains stable configuration ($d^{5}$). But in the case of Zinc ($Zn$), the first IE is very high, because we remove an electron from a stable configuration(3$\dpi{100} d^{10},4s^{2}$).
The second IE is much higher than the 1st IE. This is because it becomes difficult to remove an electron when we already did that and it already has a stable configuration (such as $\dpi{100} d^{0}\ ,d^{5}\ ,d^{10}$). For example elements such as $Cr^{+}$and $Cu^{+}$ the second IE is extremely high because they already in a stable state. And we know that removal of an electron from a stable state requires a lot of energy.

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