Gadolinium has a low value of third ionisation enthalpy because of ______.

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Gadolinium has a low value of third ionisation enthalpy because of high exchange enthalpy. Explanation: Gadolinium (Gd) has an electronic configuration [Xe] 4f7 5d1 6s2. After removing two electrons (from 6s and 5d), it has a half-filled 4f7 configuration, which is very stable. Due to this half-filled 4f subshell, it gains extra stability from exchange energy. So, less energy is needed to remove the third electron, making the third ionisation enthalpy low.

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