The electron affinity of an element is a measurable physical quantity, namely, the energy released or absorbed when an isolated gas-phase atom acquires an electron, measured in kJ/mol. We must be careful not to confuse electron affinity with electronegativity.
Affinity values can be positive or negative. A positive value is obtained when the energy created between joining an electron with an atom is released. A negative value, on the other hand, is created when energy is needed to bind the electron with the atom. So if the electronic affinity is greater than zero, energy is released, otherwise it must be supplied.
Generally we can say that a negative value is obtained when there is an exothermic or spontaneous reaction, while a negative value is obtained when the reaction comes from endothermic processes. In order to verify the different values in chemistry it is sufficient to consult the Periodic Table of elements from which it can be seen that affinity increases from left to right and decreases from top to bottom.
In chemistry most of elements conventionally have a negative electronic affinity. In practice this means that the same elements do not need energy to add an electron but otherwise they release it. Each element has a different negative electronic affinity. Generally speaking it can be said that elements that are not part of the category of metals have a higher negativity.
Electronic affinity behavior:
- The electronic affinity is influenced by the octet rule. Elements in group VII A (fluorine, chlorine, bromine, iodine, and astatine) tend to gain an electron and form -1 anions. Noble gases in group VIII A already have the full octet, so adding electrons requires a large amount of energy, but it is possible.
- Elements in group 2 from beryllium and group 12 from zinc have positive affinities, as these elements have complete s or d orbitals.
- Group 15 elements have low affinity, nitrogen even positive. The reason is that these elements enjoy a slight stability, given by the presence of an electron in each of the last three p orbitals.
- Electronic affinity, in absolute value and with rare exceptions, becomes more negative going right in the period (because it slightly decreases the radius due to the attractive force of nucleus and increases the number of electrons on the last energy level, so the atom can reach more easily the maximum stability) and becomes less negative going down along the group (both for the distance from nucleus, and for the increase of atomic number, so also of electrons that exert a repulsive force that tends to destabilize the atom). For example fluorine (top right of periodic table) has a high electron affinity because it lacks only one electron to obtain the “closed shell”, then obtained an electron and therefore reached the shell, releasing energy stabilizes.
Moreover a property related to electronic affinity is electronegativity, as atoms more electronegative will have a greater tendency to acquire electrons. This explains why fluorine has a very high electronic affinity. However, the difference in electronic affinity between chlorine and fluorine should be noted, because although fluorine is more electronegative than chlorine, the latter has a more negative electronic affinity value. This is explained by the greater atomic radius of chlorine, so the electrons that reach the outer orbitals are less affected by the repulsive force exerted by the valence electrons.
Electron affinity is not limited to elements, but also applies to molecules. For example those of benzene and anthracene are positive, that of naphthalene close to zero.