Electron affinity

The electron affinity, Eea, of an atom or molecule is the energy required to detach an electron from a singly charged negative ion, i.e., the energy change for the process
 * X- &rarr; X + e-

An equivalent definition is the energy released (Einitial &minus; Efinal) when an electron is attached to a neutral atom or molecule. It should be noted that the sign convention for Eea is the opposite to most thermodynamic quantities: a positive electron affinity indicates that energy is released on going from atom to anion.

All elements whose EA have been measured using modern methods have a positive electron affinity, but older texts mistakenly report that some elements such as alkaline earth metals have negative Eea, meaning they would repel electrons. This is not recognized by modern chemists. The electron affinity of the noble gasses have not been conclusively measured, so they may or may not have slightly negative EAs. Atoms whose anions are relatively more stable than neutral atoms have a smaller Eea. Chlorine most strongly attracts extra electrons; mercury most weakly attracts an extra electron. Eea of noble gases are close to 0.

Although Eea vary in a chaotic manner across the table, some patterns emerge. Generally, nonmetals have more positive Eea than metals.

Values for the elements
The following data are quoted in kJ/mole. Elements marked with an asterisk are expected to have electron affinities close to zero on quantum mechanical grounds.

Periodic trends
Eea increases across a period (row) (since the radius slightly decreases, because of the increased attraction from the nucleus, and the number of electrons in the top shell increases, helping the atom reach maximum stability) in the periodic table and decrease going down a group (family) (because of a large increase in radius and number of electrons that decrease the stability of the atom, repulsing each other).

Molecular electron affinities
Eea is not limited to the elements but also applies to molecules. For instance the electron affinity for benzene is negative, as is that of naphthalene, while those of anthracene,phenanthrene and pyrene are positive. In silico experiments show that the electron affinity of hexacyanobenzene surpasses that of fullerene.