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Energy release in electron capture reactions: Why is this an amazing process?
In the fields of chemistry and physics, electron affinity (Eea) is defined as the energy released when an electron attaches to a neutral atom or molecule. The reaction in the gas state can be expressed as:
X(g) + e− → X−(g) + energy
During the electron capture process, the energy released makes many atomic and molecular interactions more stable. This process is invisible in our daily lives, but is a vital part of basic science. For example, this phenomenon has different definitions in solid-state physics, and this difference has led to a new level of understanding of electron affinity.
Measurement and application of electron affinity
Measurements of electron affinity are limited to atoms and molecules in the gaseous state because in the solid or liquid state, energy levels change when they come into contact with other atoms or molecules. This property makes electron affinity a precise measurement tool. Renowned chemist Robert S. Mulliken used data on electron affinity to develop a standard for the electronegativity of atoms:
Electronegativity is equal to the average of electron affinity and ionization energy.
In addition, electron affinity is also involved in the discussion of theoretical concepts such as electron chemical potential and chemical hardness. In chemical reactions, atoms with higher electron affinity are often called electron acceptors, while electron donors are those with lower affinity, and charge transfer reactions may occur between the two.
Signature Specifications
Correct use of electron affinity requires attention to its sign. For example, for a reaction that releases energy, the value of the total energy change ΔE is negative, and such a reaction is called an exothermic process. Almost all electron capture of non-noble gas atoms involves the release of energy and is therefore an exothermic process. The positive values listed in various references are actually what we call "released" energy, thus providing the negative sign for ΔE. For many people, it is confusing to misinterpret EEA as an energy change, when the actual relationship is:
Eea = −ΔE(attach)
If the value of Eea has a negative sign, it means that energy is required to attach the electron, making electron capture an endothermic process. This negative value usually occurs in the capture of a second electron, or in nitrogen atoms.
Electron Affinity of Elements
Although electron affinities vary across the periodic table, we can still observe some trends. Generally speaking, the Eea value of non-metals will be higher than that of metals. When the anion is more stable than the neutral atom, the value of Eea will be larger. For example, chlorine has the strongest attraction for extra electrons, while neon has the weakest. The electron affinities of the noble gases are not well defined, so their values may be negative.
Normally, Eea increases in order across the rows (horizontally) of the periodic table. In Group 17, as atoms gain electrons to fill up the valence band, the energy released increases. Although many people would expect the electron affinity to decrease as the period progresses downward, in fact, in many columns, the Eea actually increases.
Molecular Electron Affinity
The electron affinity of a molecule is a more complex function that is affected by its electronic structure. For example, benzene has a negative electron affinity, while anthracene, phenanthrene, and pyrene have positive values. In addition, the calculation results also show that the electron affinity of hexacyanobenzene exceeds that of fullerene.
Electron Affinity in Solid State Physics
In solid-state physics, electron affinity is defined differently. For the interface between a semiconductor and a vacuum, the electron affinity is defined as the energy gained by moving an electron from the vacuum to the bottom of the conduction band inside the semiconductor. In a semiconductor at absolute zero, this concept is similar to the definition of electron affinity in chemistry. However, at temperatures above absolute zero, and for other materials such as metals and heavily doped semiconductors, the added electrons will generally go to the Fermi level rather than the bottom of the conduction band.
How to effectively use these electron affinities and their measurement will become an important consideration in modern materials science and physics. This knowledge will guide us whenever we explore surface terminations, cutoff structures and their effects. Can you imagine how future technology might benefit from a greater understanding of electron affinity?
