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Why is first ionisation enthalpy of Cr l...

Why is first ionisation enthalpy of `Cr` lower than that of `Zn` ?

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The electronic configuration of the two metals are :
`Cr(Z=24)[Ar]4s^(1)3d^(5)`
`Zn(Z=30)4s^(2)3d^(10)`
The `Delta_(i)H_(1)` value of `Cr` `(652 kJmol^(-1))` is lower than that of `Zn` `(906 kJ mol^(-1))` due to different reasons.
In case of `Cr`, after losing one `4s^(1)` electrons, the cation had `3d^(5)` configuration. However, in case of `Zn`, the effective nuclear charge is quite large. The electron has to be removed from filled `4s` orbital which has more penetration in the nucleus as compared to `3d` orbital. The `Delta_(i)H_(1)` value of `Cr` is therefore, lower than that of `Zn`.
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