Question

Electron gain enthalpy of group 18 elements is 0 or positive. Why?

Answer 1

Hint: The electron gain enthalpy of an element can be thought of as a measure of its willingness to accept an electron. Therefore, the electron gain enthalpy of an element will depend on its electronic configuration.

Complete Step by Step Solution:
Electron gain enthalpy (sometimes denoted as \[{\Delta _{eg}}H\]) is defined as the amount of energy absorbed or released when an electron is added to an isolated, gaseous atom. According to the sign convention, when energy is absorbed by adding an electron to an atom of a particular element, the electron gain enthalpy of that element is said to be positive and if energy is released by adding an electron, the electron gain enthalpy is said to be negative.

Group 18 elements are non-metals. Since nonmetals generally prefer to accept electrons, we will be discussing electron gain enthalpy in terms of negative electron gain enthalpy only. When expressed with a negative sign, the electron gain enthalpy gives us an idea of how much an atom of an element favours accepting an electron. A high negative electron gain enthalpy indicates that the atom favours accepting an electron and a low electron gain enthalpy indicates that the atom does not favour accepting an electron.
Group 18 elements have a 0 or even a positive electron gain enthalpy. This indicates that they strongly disfavour accepting an electron into their valence shell.

Group 18 elements (also called noble gases) have the valence shell electronic configuration \[n{s^2}{\rm{ n}}{{\rm{p}}^6}\]. This means that their valence orbitals are fully filled which corresponds to the highest stability configuration. If another electron is added to this, it will make the atom unstable (because the highest stability configuration will get disturbed) which is why atoms of noble gases will heavily oppose the addition of this extra electron. The heavy opposition manifests as a 0 or positive electron gain enthalpy in noble gases.
Thus, the electron gain enthalpies of Group 18 elements are 0 or positive because of their stable electronic configurations.

Note: Some sources may use the terms “electron gain enthalpy” and “electron affinity” synonymously. Although both terms are related, they are different from each other. Electron affinity is generally the negative of electron gain enthalpy. The electron affinities of noble gases are negative (opposite to that of electron gain enthalpy).