Gadolinium has a low value of third ionisation enthalpy because of high exchange enthalpy.

Electronic configuration of Gadolinium [Gd) = [Xe] 4f 7 5d 1 6s 2

For 3 rd ionisation enthalpy, electron will be removed from 5d,

So the resultant configuration will be [Xe]4f 7 that is stable electronic configuration as it will have high exchange energy.

Therefore, less energy will be required to remove 3 rd electron

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