Question

Give reasons of the following:
a. Electron gain enthalpies of halogens are largely negative.

Answer 1

Halogens are known to us as the most electronegative elements in the periodic table for having electron gain enthalpies that are largely negative.

Complete solution
a. In the periodic table, we know that the elements of group $17$ are known as halogens. They include fluorine, chlorine, bromine, iodine and astatine. As we know that, outer shell electronic configuration for all the elements is the same in a group, it is true for halogens as we can write the general valence shell electronic configuration for halogens to be $n{s^2}n{p^5}$. It is evident that they require just one more electron to have a highly stable noble gas configuration i.e. $n{s^2}n{p^6}$. This can be done by halogens with adding up an extra electron that would result in the formation of negatively charged halide anion.
Now, we all know that it is the enthalpy change that accompanies addition of an electron to a neutral gaseous atom that we know as electron gain enthalpy. We can represent the electron gain enthalpy with the symbol ${\Delta _{eg}}H$. We can have positive as well as negative electron gain enthalpies depending upon how readily a gaseous atom of an element adds up an electron. If it is an easy and spontaneous process, we will have negative electron gain enthalpy. However, if it is a challenging and non-spontaneous process then we will have positive electron gain enthalpy.
Therefore we have seen that it is the easiest to add an electron to the gaseous atoms of halogens as that would give them higher stability and hence, we can say that they have large negative electron gain enthalpies.

Note:
The readiness of halogens to add an extra electron can be related to their ability to pull the electron density towards themselves that makes them highly electronegative.