Electron Affinity

When an electron is added to a gaseous atom, a change in energy is observed called electron affinity. Electron affinity measures the ease of gaining an electron by an atom.

For example, the electron affinity of chlorine is −348.6 kJ/mol. The negative sign indicates that it is an exothermic change.

Argon, however, has a positive electron affinity, indicating that formation of an argon anion requires energy to be supplied.

In general, the greater the attraction between an atom and an added electron, the more negative the electron affinity. Electron affinities, similar to ionization energies, show trends in the periodic table.

Moving down group 1, the atomic size increases as the electrons occupy higher principal quantum numbers. Incoming electrons, therefore, experience less nuclear attraction leading to less negative electron affinities.

However, there are exceptions. In halogens, chlorine has a more negative electron affinity value than fluorine. But why?

Fluorine is the smallest atom of the halogens and an incoming electron experiences a significant repulsion from the electrons already present.

In the chloride anion, however, the new electron is added into the third shell, occupying more space. This reduces the electron-electron repulsions, making it more attractive for an electron to be gained.

Generally, moving across a period, electron affinities become more negative. Halogens have the most negative electron affinities, as the incoming electron helps to achieve noble gas configurations.

In comparison, noble gases have a completely filled shell. The incoming electron has to be accommodated in the higher principal energy level, which is energetically unfavorable. Thus, electron affinities for these elements are positive.

Group 2 shows exceptions. The electron configuration indicates that the incoming electron needs to enter a higher-energy subshell. Thus, electron affinity values are either positive or less exothermic.

Interestingly, group 15 has less negative electron affinities than group 14. Compare phosphorus and silicon. Unlike silicon, phosphorus has a half-filled p-subshell and the incoming electron needs to be paired with an electron already residing in the p-orbital.

This would increase the electron-electron repulsions and is therefore an energetically unfavourable process, which is also reflected in the less negative electron affinity compared to silicon.