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Question
Why first ionisation enthalpy of Cr is lower than that of Zn?
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Solution
The electronic configuration of chromium and zinc are respectively:
\[\ce{Cr (24) = [Ar] 3d^5 4s^2}\]
\[\ce{Zn (30) = [Ar] 3d^10 4s^2}\]
It is easy to remove electron from 4s1-orbital (unpaired) rather than from 4s2-orbital (paired).
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