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6:18 AM
Why does He(helium) has less positive electron gain enthalpy than Neon(Ne). The size of atoms increase down the zero group and also the effective nuclear charge increase down the group. Therefore the electron is attracted more towards the nucleus and hence the electron gain enthalpy should become less positive down the zero group. BUT in fact helium has the least positive electron gain enthalpy in the zero group!!

What is the correct explanation for the observed trend?
 

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