1.4.3: Electron Affinity
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- Kathryn Haas
- Duke University
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
- 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.
- Electron Affinity (data page), Wikipedia. en.Wikipedia.org/wiki/Electron_affinity_(data_page) Accessed 12/3/19.