Electron Affinity in Modern Periodic Table

Introduction

The electron affinity of an atom or molecule represents the ability of that particle to gain an electron. Electron affinity is an exothermic process for all non-noble gas elements. There are some general trends in electron affinity across and down the periodic table of elements. Electron affinity usually increases across a period in the periodic table and sometimes it decreases down a group. These trends are generally not universal. The chemical rationale for changes in electron affinity across the periodic table represents the increased effective nuclear charge across a period and up a group.

What is Electron Affinity?

Electron affinity is referred to as the change in energy (in kJ/mol) of a neutral atom (in the gaseous phase) when an electron is added to the atom to form a negative ion. In other words, the neutral atom is capable of gaining an electron.

Energy of an atom can be defined when the atom loses or gains energy via chemical reactions that result in the loss or gain of electrons. A chemical reaction that releases energy is known as an exothermic reaction and a chemical reaction that absorbs energy is known as an endothermic reaction. Energy from an exothermic reaction is always negative, therefore, energy is given a negative sign; while, energy from an endothermic reaction is always positive and energy is written in a positive sign. An example that represents both these processes is when a person drops a book. When one lifts a book, or they gives potential energy to the book (energy is absorbed). However, once the person drops the book, the potential energy converts itself into kinetic energy and this is released in the form of sound once it hits the ground (energy released).

When an electron is added to any neutral atom (i.e., first electron affinity) energy is released; therefore, the first electron affinities are always negative. Although, more energy is required to add an electron to a negative ion (i.e., second electron affinity) that overwhelms any release of energy from the electron attachment process and hence, second electron affinities are always positive.

First Electron Affinity (negative energy since energy is released): X(g)+e−→X−(g)

Second Electron Affinity (positive energy because energy needed is more than gained): X−(g)+e−→X2−(g)

Periodic Trends in Electron Affinity

However, Eea differs greatly across the periodic table, and some patterns emerge. Usually, non-metals possess more positive Eea than metals. Atoms, like those of Group 17 elements, whose anions are more stable than the neutral atoms possess a higher Eea. The electron affinities of the noble gases have yet not been conclusively measured, that is why they may or may not have slight negative values. Chlorine possess the highest Eea while mercury has the lowest.

Eea usually increases across a period (i.e. row) in the periodic table, because of the filling of the valence shell of the atom. For example, within the same period, a Group-17 atom releases more energy than a Group-1 atom due to gaining of an electron as because the added electron creates a filled valence shell and thus is more stable.

Therefore a trend of decreasing Eea down the groups in the periodic table would be expected, as the additional electron is entering an orbital farther away from the nucleus. As this electron is farther away, it should be less attracted to the nucleus and thus release less energy when added. Moreover, this trend is applicable only to Group-1 atoms. Electron affinity follows the trend of electronegativity: fluorine (F) possesses a higher electron affinity than oxygen (O), and so on. The trends noted here are very similar to those in ionization energy.

Factors Affecting Electron Affinity

The stronger the attraction, the more energy is released, and the higher will be the electron affinity. The major two factors that influence these trends,  include atomic size and nuclear charge. Nuclei possess greater positive charge and attract electrons more strongly, thereby resulting in a larger electron affinity. Conversely, less positive nuclear charges leads to a smaller electron affinity.

Regarding atomic size, smaller atoms usually offer less space for electrons to gather, including the incoming electron. As a result, this extra electron will position itself close to the nucleus than it would in a larger atom. This leads to a  larger electron affinity value for smaller atoms due to the increased attraction between this incoming electron and the nucleus. On the other hand, larger atoms tend to show smaller electron affinities since they offer more space for electrons to share with themselves as well as the incoming electron. 

But we must not overlook the effects of repulsion and shielding on electron affinity. As, smaller atoms may also exhibit more attraction and larger electron affinities, but due to their lack of space for gathering electrons also results in an increased repulsion between these particles. Repulsion lessens as the attraction between the incoming electron and the nucleus facilitates lowered electron affinities. The shielding effect acts in a similar manner.

We must consider each of these factors carefully, because their effects may vary based on the characteristics of each element.  

Conclusion

Chlorine possess the highest electron affinity among all the elements. Its high affinity is mainly due to its large atomic radius, or size. Since, the chlorine outermost orbital is 3p, its electrons possess a large amount of space to share with an incoming electron. This reduces the repulsions between these particles to a degree that overlooks the negative effects of its large size on attraction. Mercury possesses the lowest electron affinity .