(solved)Question 3.20 of NCERT Class XI Chemistry Chapter 3

Enthalpy change associated with the addition of an electron to a neutral gaseous atom to form a negative ion is called electron gain enthalpy.

Sign of Electron Gain Enthalpy

by convention, the sign is taken negative if the electron gain is an exothermic process that releases energy.

It is more negative if more energy is released.

It is positive for noble gases because the process is endothermic. Some metals like Hg, Cd, Be, Mg, Zn, Mn and even the nitrogen gas have positive electron gain enthalpies.

General trend for Metals vs non-Metals

(1) Non-metals usually have negative electron gain enthalpies because they have a larger number of valence electrons that can gain additional electrons to have a stable octet.

(2) Non-metals have comparatively smaller atomic sizes with greater effective nuclear charge to favor the acceptance of electrons, and make the process more exothermic.

(3) Metals can easily lose their valence electrons. Hence, they have less negative electron gain enthalpies. Some even have positive electron gain enthalpies.

(4) The general conclusion is that the metals have lesser affinity towards electrons as compared to non-metals.

General trends along a Period

Electron gain enthalpy increases from left to right.

Reason 1: the atomic size decreases and the effective nuclear charge increases from left to right in a period. This causes the electron gain to be a more exothermic process.

Reason 2: the valence electrons increase from left to right in a period. Hence addition of an electron makes it more likely to form a stable octet.

General trends down a Group

Atomic radii tend to increase as we move down a group. There is an increased shielding as we move down a group. As a result, the attraction between the nucleus and the valence electrons reduces, leading to a less negative electron gain enthalpy as we move down a group.

Anomaly of Florine (Chlorine) and Oxygen (Sulfur)

Electron gain enthalpy should become less negative down a group but florine has less negative as compared to its successor chlorine, and oxygen, too, also has less negative than sulfur.

The reason for this anomaly is that florine atom is so small that an additional electron experiences a significant amount of repulsion from other electrons. This repulsion reduces the electron gain enthalpy.

The same reasoning applies to the oxygen atom.

Why is Florine more reactive than Chlorine?

Electron gain enthalpy of chlorine is more negative than that of florine, so it might suggest that chlorine will more readily form a negative ion, and hence be more reactive.

But in reality, florine has been found to be much more reactive than chlorine.

The reason is that a chemical reaction is made of many intermediate steps. The energy released in other steps happens to more than compensate for the difference in electron gain enthalpies.

Why is second Electron Gain Enthalpy of Oxygen very highly positive?

Oxygen atom is very small. The atom becomes very electron-dense after first electron gain. As a result, adding a second electron is an endothermic process that requires a large amount of energy.