Electron affinity

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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 gases 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 greater 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.

1 Values for the elements
2 Periodic trends
3 Molecular electron affinities
4 See also
5 References
6 External links

Values for the elements

Main article: Electron affinity (data page)

The following data are quoted in kJ/mol. Elements marked with an asterisk are expected to have electron affinities close to zero on quantum mechanical grounds. Elements marked with a dotted box are synthetically made elements - elements not found naturally in the environment.

Group →	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18
↓ Period
1	H 73																	He *
2	Li 60	Be *											B 27	C 122	N *	O 141	F 328	Ne *
3	Na 53	Mg *											Al 42	Si 134	P 72	S 200	Cl 349	Ar *
4	K 48	Ca 2	Sc 18	Ti 8	V 51	Cr 65	Mn *	Fe 15	Co 64	Ni 112	Cu 119	Zn *	Ga 41	Ge 119	As 79	Se 195	Br 343	Kr *
5	Rb 47	Sr 5	Y 30	Zr 41	Nb 86	Mo 72	Tc *	Ru 101	Rh 110	Pd 54	Ag 126	Cd *	In 39	Sn 107	Sb 101	Te 190	I 295	Xe *
6	Cs 46	Ba 14	*	Hf	Ta 31	W 79	Re *	Os 104	Ir 150	Pt 205	Au 223	Hg *	Tl 36	Pb 35	Bi 91	Po	At	Rn *
7	Fr	Ra	**	Rf	Db	Sg	Bh	Hs	Mt	Ds	Rg	Uub	Uut	Uuq	Uup	Uuh	Uus	Uuo

* Lanthanides				La 45	Ce 92	Pr	Nd	Pm	Sm	Eu	Gd	Tb	Dy	Ho	Er	Tm 99	Yb	Lu 33
Actinides**				Ac	Th	Pa	U	Np	Pu	Am	Cm	Bk	Cf	Es	Fm	Md	No	Lr

**Chemical series of the periodic table**
Alkali metals	Alkaline earth metals	Lanthanides	Actinides	Transition metals
Poor metals	Metalloids	Nonmetals	Halogens	Noble gases

Periodic trends

E_ea 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 7A atom releases more energy than a group 1A atom on gaining an electron because it obtains a filled valence shell.

A trend of decreasing E_ea going down the groups in the periodic table would be expected. The additional electron will be entering an orbital farther away from the nucleus, and thus would experience a lesser effective nuclear charge. However, a clear counterexample to this trend can be found in group 2A, and this trend only applies to group 1A atoms.

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 ^[1].

References

^ Remarkable electron accepting properties of the simplest benzenoid cyanocarbons: hexacyanobenzene, octacyanonaphthalene and decacyanoanthracene Xiuhui Zhang, Qianshu Li, Justin B. Ingels, Andrew C. Simmonett, Steven E. Wheeler, Yaoming Xie, R. Bruce King, Henry F. Schaefer III and F. Albert Cotton Chemical Communications, 2006, 758 - 760 Abstract