Structural Biochemistry/Electron Affinity

Definition

Electron affinity is the energy change or gain that is accompanies an atom when an electron is added to it. When a neutral chlorine atom in the gaseous form picks up an electron to form a Cl- ion, it releases an energy of 349 kJ/mol or 3.6 eV/atom. It is said to have an electron affinity of -349 kJ/mol and this large number indicates that it forms a stable negative ion. Small numbers indicate that a less stable negative ion is formed. Groups VIA and VIIA in the periodic table have the largest electron affinities. Alkali earth elements (Group IIA) and noble gases (Group VIIIA) do not form stable negative ions.

The sign of the electron affinity is associated with the potential energy that is created with the addition of the electron. If the addition of the electron were to create stability within the atom, it is considered to have higher electron affinity, which decreases the potential energy and this energy gained is negative. If the addition of the electron were to make the atom less stable, the potential energy increases, and the energy gained is found to be negative.

Electron affinity is essentially the opposite of the ionization energy. Instead of removing an electron from the element, we add an electron to the element to create an anion. Also, the noble gases, alkali metals and alkali earth metals have electron affinity.

Trends

Across the Period

Typically, electron affinity increases from left to right across the period, but it is definitely not a regular or steady increase. Factors such as charge and the atomic size both affect electron affinities, hence the trend across the period is not as regular as the other periodic table trends.

Across the Group

Electron affinity tends to decrease down a group, but as mentioned previously, it is not a regular trend, and there are several exceptions to this rule of thumb down the group.

References

1. Silberberg, Martin S. Principles of General Chemistry. Boston: McGraw-Hill Higher Education, 2007. Print.