Question

why the noble gases have highly positive electron gain enthalpy

Answer 1

hi!
Noble gases have stable ns2 np6 fully filled electronic configuration so, these have completely filled octet. Thus they have no tendency to add electron and the added electron experiences a repulsion.As a result the electron gain enthalpy is high.
In order of increasing electron gain enthalpy the noble gases are arranged as follows:
Xe< Kr<Ar< Ne<He
He being the smallest ,offers maximum repulsion to the added electron, thus its electron gain enthalpy is high followed by Ne , Ar, Kr and Xe.