Editing Electron affinity (section)

== Measurement and use of electron affinity ==

This  property is used to measure atoms and molecules in the gaseous state only, since in a solid or liquid state their [[energy level]]s 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 electrons
affinity and [[ionization potential]].<ref>Robert S. Mulliken, [[Journal of Chemical Physics]], '''1934''', ''2'', 782.</ref><ref>Modern Physical Organic Chemistry, Eric V. Anslyn and Dennis A. Dougherty, University Science Books, 2006, {{ISBN|978-1-891389-31-3}}</ref> 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 [[Intervalence charge transfer|charge-transfer]] reactions.

=== Sign convention ===
To use electron affinities properly, it is essential to keep track of sign. For any reaction that ''releases'' energy, the ''change'' Δ''E'' in [[total energy]] has a negative value and the reaction is called an [[Exothermic reaction|exothermic process]]. Electron capture for almost all non-[[noble gas]] atoms involves the release of energy<ref>Chemical Principles the Quest for Insight, Peter Atkins and Loretta Jones, Freeman, New York, 2010 {{ISBN|978-1-4292-1955-6}}</ref> and thus is exothermic. The positive values that are listed in tables of ''E''<sub>ea</sub> are amounts or magnitudes. It is the word "released" within the definition "energy released" that supplies the negative sign to Δ''E''. Confusion arises in mistaking ''E''<sub>ea</sub> for a change in energy, Δ''E'', in which case the positive values listed in tables would be for an endo- not exo-thermic process. The relation between the two is ''E''<sub>ea</sub> = −Δ''E''(attach).

However, if the value assigned to ''E''<sub>ea</sub> 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, ''E''<sub>ea</sub> = −Δ''E''(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 ''E''<sub>ea</sub> when an electron is attached is

:{{math|size=120%|1=''E''<sub>ea</sub> = (''E''<sub>initial</sub> − ''E''<sub>final</sub>)<sub>attach</sub> = −Δ''E''(attach)}}

This expression does follow the convention Δ''X'' = ''X''(final) − ''X''(initial) since −Δ''E'' = −(''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 the atom while it holds a [[Electric charge|single-excess-electron]] thus making the atom a [[ion|negative ion]],<ref name="Compendiumof">{{GoldBookRef|title=Electron affinity|file=E01977}}</ref> i.e. the energy change for the process

:X<sup>−</sup> → X + e<sup>−</sup>

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 Δ''E''(detach) > 0.

:{{nowrap|{{math|size=120%|1=''E''<sub>ea</sub> = (''E''<sub>final</sub> − ''E''<sub>initial</sub>)<sub>detach</sub> = Δ''E''(detach) = −Δ''E''(attach)}}.}}