According to NCERT book electron gain enthalpy of group 18 should beHe +48Ne +116Ar +96Kr +96Xe +77Rn +68As we go down in a group electron gain enthalpy decrease. It becomes less negative (more positive). In group 18 it becomes less positive. It means that we have to put less energy as we go down the gruop. The size of the last element in group 18 is more than other members so it will have a larger orbit so the electron which is coming will get a little attraction of protons. Then why we have to put less energy in that ?

Dear Student

Electron gain enthalpy is defined as the energy change taking place(energy evolved or required) when an electron is added to a gaseous neutral atom to form a monovalent anion.
Since it is not easy for any noble gas atom to accept an electron so we need to provide energy from outside to add an electron to the atom.
And thus it is the reason that the EGE of noble gases will be positive.
Now as you go down the group, due to more number of d and f electrons in the atom, the effective nuclear charge increases and thus it will be comparatively easy to add an extra electron to the atom and thus less energy will be required and so less positive EGE

Hope it helps!!

Approved