# 2.3.2: Electron Affinity

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## Definitions of Electron Affinity

According to IUPAC, there are two different, but equivalent, definitions of electron affinity (EA).1

##### Definition: Electron Affinity defined as removal of an electron

Electron affinity can be defined as the energy required when an electron is removed from a gaseous anion. The reaction as shown in equation $$\ref{EA1}$$ is endothermic (positive $$\Delta U$$) for elements except noble gases and alkaline earth metals. Under this definition, the more positive the EA value, the higher an atom's affinity for electrons.

$A^{-}_{(g)} \longrightarrow A_{(g)} + e^- \hspace{1cm} EA = \Delta U \label{EA1}$

The reaction shown in Equation $$\ref{EA1}$$ is similar those that define ionization energy. For this reason, the EA is also described as the zeroth ionization energy.

##### Definition: Electron Affinity defined as addition of an electron

An alternate and more common definition is the microscopic reverse of Equation $$\ref{EA1}$$. This more common definition states that electron affinity is the energy released when an electron is added to a gaseous atom, as shown in Equation $$\ref{EA2}$$. The reaction as shown in equation $$\ref{EA2}$$ is exothermic (negative $$\Delta U$$) for elements except noble gases and alkaline earth metals. The more negative this EA value, the higher an atom's affinity for electrons.

$A_{(g)} + e^- \longrightarrow A^{-}_{(g)} \hspace{1cm} EA = \Delta U \label{EA2}$

Conceptually, this second definition is quite similar to the concept of electronegativity; but unlike electronegativity, EA is a well-defined quantitative measurement.

## Trends in Electron Affinity

For this discussion, we will use the definition of EA that is consistent with it being a zeroth ionization energy: a more positive (larger) value means that the EA is higher (meaning stronger affinity toward an electron).

• Across a period: Similar to ionization energy, EA generally increases across a row of the periodic table; this observation is consistent with the increase in effective nuclear charge (Z*) from left to right across a period. However, there are variations across a period that are similar to variations in ionization energy and that can be explained by shielding, penetration, and electron configuration.
• Down a group: Like the case of ionization energy trends, EA does not consistently decrease going down a column of the periodic table despite the fact that $$Z^*$$ increases down a group.

The trend in EA follows a zig-zag pattern similar to the one seen with ionization energies, except that it is displaced by one unit from the trend in $$I_1$$, two units from $$I_2$$, and so on. For example, EA peaks at F, while $$I_1$$ peaks at Ne, $$I_2$$ peaks at Na, and $$I_3$$ peaks at Mg. A plot of EA for the first 13 elements is shown overlaid on plots of $$I_1, I_2$$ and $$I_3$$ in Figure $$\PageIndex{1}$$., where the shifts in the peaks and valleys within each zig-zag trend are indicated.

## Sources

1. IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). Online version (2019-) created by S. J. Chalk. ISBN 0-9678550-9-8. doi.org/10.1351/goldbook.
2. Electron Affinity (data page), Wikipedia. en.Wikipedia.org/wiki/Electron_affinity_(data_page) Accessed 12/3/19.

This page titled 2.3.2: Electron Affinity is shared under a CC BY-NC 4.0 license and was authored, remixed, and/or curated by Kathryn Haas.