The electron affinity (Eea) of a neutral atom or molecule is defined as the amount of energy released when an electron is added to it to form a negative ion, as demonstrated by the following equation:
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. Robert S. Mulliken used a list of electron affinities to develop an electronegativity scale for atoms by finding the average of the electron affinity and ionization potential. A molecule or atom that has a more positive electron affinity value is often called an electron acceptor; one with a less positive electron affinity is called an electron donor. Together they may undergo charge-transfer reactions.
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 therefore is an exothermic process.
Confusion may arise in mistaking Eea for change in energy, ΔE.The numbers listed in tables of Eea are all positive because they are magnitudes; it is the "release," or loss, of energy that tells us we need a negative sign. Restated, the values you see when looking up a table of electron affinities all indicate the amount of energy released when an electron is added to an element. Because this is an exothermic event, translating these numbers to ΔE would result in negative values (indicating an exothermic process).
Periodic Trends in Electron Affinity
Although Eea varies greatly across the periodic table, some patterns emerge. Generally, nonmetals have more positive Eea than metals. Atoms whose anions are more stable than neutral atoms have a greater Eea. Chlorine most strongly attracts extra electrons; mercury most weakly attracts an extra electron. The electron affinities of the noble gases have not been conclusively measured, so they may or may not have slightly negative values.
Eea generally increases across a period (row) in the periodic table. This is caused by the filling of the valence shell of the atom; a Group-17 atom releases more energy than a Group-1 atom upon gaining an electron because it obtains a filled valence shell and therefore is more stable.
A trend of decreasing Eea down the groups in the periodic table would be expected, since the additional electron is entering an orbital farther away from the nucleus. Since this electron is farther away, it should be less attracted to the nucleus and release less energy when added. However, this trend applies only to Group-1 atoms. Electron affinity follows the trend of electronegativity: fluorine (F) has a higher electron affinity than oxygen, and so on.
The trends noted here are very similar to those in ionization energy and indeed change for similar (though opposing) reasons.