Answer to Question #330595 in General Chemistry for Liza

Question

Elements with very large ionization energies also tend to have highly exothermic electron affinities. Explain. Which group of elements would you expect to be an exception to this statement?

Answer 1

Ionisation energy is the energy required to be supplied in order that the most losely held electron from a gaseous atom can be taken out and moved to infinity. The value of this energy is always positive meaning that it has to be supplied to the atom and it is an indicator of how strong is the effective nuclear charge at the shell/orbit from which the electron is to be extracted. This in turn, represents the energy level of that shell/orbit. And, as we move from left to right along a period or from bottom to top in a group, the effective nuclear charge increases due to decrease in atomic size and simultaneously, the energy required to take out the valence electron (and so the ionisation energy) increases. At the same time, the amount of energy released when a gaseous atom accepts an electron into it's valence shell increases with effective nuclear charge as increase in effective nuclear charge would simply mean that, any electron entering the valence shell should be pulled into the atom with greater force and result in the release of more energy. This effectively results in more negative electron gain enthalpy (or more exothermic electron affinity).

Hence, as we move right along a period or up in a group, the effective nuclear charge on the valence electrons increases due to decrease in atomic size. This on one hand, causes increase in the energy needed to extract a valence electron (ionisation enthalpy), while on the other, causes more energy to be released (highly exothermic electron affinity) when an electron is inserted into it's valence shell.

The only known exceptions that come to my mind to this are Oxygen and Fluorine. Both occupy the topmost and the rightmost positions, one after another, and, essentially have the smallest sizes with very high effective nuclear charges. This results in very high ionisation enthalpies for both. However, their electron affinities are less exothermic than their succeeding group members (-141 kJ per mole for Oxygen while -200 kJ per mole for Sulphur and -328 kJ per mole for Fluorine while -349 kJ per mole for Chlorine) because their small size makes it difficult for each of the atom's valence shell to stabilize itself after accepting an electron due to repulsion from the other electrons already present in it.