Electron affinity

The electron affinity, E_ea, 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^- → X + e^-

An equivalent definition is the energy released (E_initial − E_final) when an electron is attached to a neutral atom or molecule. It should be noted that the sign convention for E_ea 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 E_ea, 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 E_ea. Chlorine most strongly attracts extra electrons; mercury most weakly attracts an extra electron. E_ea of noble gases are close to 0.

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

Values for the elements

Main article: Electron affinity (data page)

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.

Group →	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18
↓ Period
1	Template:Element cell property																	Template:Element cell property
2	Template:Element cell property	Template:Element cell property											Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property
3	Template:Element cell property	Template:Element cell property											Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property
4	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property
5	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property
6	Template:Element cell property	Template:Element cell property	Template:Element cell	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property
7	Template:Element cell property	Template:Element cell property	Template:Element cell	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property
* Lanthanides				Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property
** Actinides				Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property	Template:Element cell property

Chemical series of the periodic table

style="background:Template:Element color/Alkali metals;" width="20%" | Alkali metals	style="background:Template:Element color/Alkaline earth metals;" width="20%" | Alkaline earth metals	style="background:Template:Element color/Lanthanides;" width="20%" | Lanthanides	style="background:Template:Element color/Actinides;" width="20%" | Actinides	style="background:Template:Element color/Transition metals;" width="20%" | Transition metals
style="background:Template:Element color/Poor metals;" | Poor metals	style="background:Template:Element color/Metalloids;" | Metalloids	style="background:Template:Element color/Nonmetals;" | Nonmetals	style="background:Template:Element color/Halogens;" | Halogens	style="background:Template:Element color/Noble gases;" | Noble gases

Periodic trends

E_ea 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

E_ea 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 .

See also

• Koopmans' theorem
• One-electron reduction
• Ionization potential
• Electronegativity
• Valence Electrons

External links

• Electron affinity, definition from the IUPAC Gold Book