The electron affinity of an atom or molecule is defined as the amount of energy released when an electron is added to a neutral atom or molecule to form a negative ion.
- X + e− → X−
This property is measured for atoms and molecules in the gaseous state only, since in the solid or liquid states their energy levels would be changed by contact with other atoms or molecules. A list of the electron affinities was used by Robert S. Mulliken to develop an electronegativity scale for atoms, equal to the average of the electron affinity and ionization potential. Other theoretical concepts that use electron affinity include electronic chemical potential and chemical hardness. Another example, a molecule or atom that has a more positive value of electron affinity than another is often called an electron acceptor and the less positive an electron donor. Together they may undergo charge-transfer reactions.
In solids, the electron affinity is the energy difference between the vacuum energy and the conduction band minimum.
To use electron affinities properly, it is essential to keep track of sign. For any reaction that releases energy, the change in energy, ΔE, has a negative value and the reaction is called an exothermic process. Electron capture for almost all non-noble gas atoms involves the release of energy and thus are exothermic. The positive values that are listed in tables of Eea are amounts or magnitudes. It is the word, released within the definition energy released that supplies the negative sign. Confusion arises in mistaking Eea for a change in energy, ΔE, in which case the positive values listed in tables would be for an endo- not exo-thermic process. The relation between the two is Eea = - ΔE(attach).
However, if the value assigned to Eea is negative, the negative sign implies a reversal of direction, and energy is required to attach an electron. In this case, the electron capture is an endothermic process and the relationship, Eea = - ΔE(attach) is still valid. Negative values typically arise for the capture of a second electron, but also for the nitrogen atom.
The usual expression for calculating Eea when an electron is attached is
- Eea = (Einitial − Efinal)attach = - ΔE(attach)
This expression does follow the convention ΔX = X(final) - X(initial) since - ΔE = - (E(final) - E(initial)) = E(initial) - E(final).
Equivalently, electron affinity can also be defined as the amount of energy required to detach an electron from a singly charged negative ion, i.e. the energy change for the process
- X− → X + e−
If the same table is employed for the forward and reverse reactions, without switching signs, care must be taken to apply the correct definition to the corresponding direction, attachment-(release) or detachment-(require). Since almost all detachments (require +) an amount of energy listed on the table, those detachment reactions are endothermic, or ΔE(detach) > 0.
- Eea = (Efinal − Einitial)detach = ΔE(detach) = - ΔE(attach)
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