Question

The electron affinities (as delta H values) of chlorine, bromine, and iodine (as gaseous atoms) are -349, -324, and -295 kJ/mol. Based on the electron affinity trend, what would you expect the electron affinity of fluorine (gaseous atom) to be? The electron affinity of fluorine is actually -328 kJ/mol. Suggest an explanation for the difference predicted and actual.

Answer 1

Electron affinity is the tendency of an element to attract the valence electrons of the other elements. Since halogens have one electron vacancy in their outermost shell, they have high electron affinity values. On moving down the group, this tendency keeps on decreasing due to the increase in atomic size. The trend followed by the halogens for electron affinity is as follows:

I < Br < F < Cl

Here it can be thought that Fluorine should have the highest electron affinity than Chlorine but that is not the case. Fluorine has a small atomic size and high electronegativity due to which it has a strong pull on its own electrons. Due to its small size, the other element's electrons when pulled by Fluorine result in electron repulsions in the atom and this is the reason why it has a lower electron affinity than Chlorine.

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