Electron affinity

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Electron affinity is the measure of the energy change when an electron is added to a neutral atom to form a negative ion., E_ea, of an atom or molecule is the amount of energy required to detach an electron from a singly charged negative ion,^[1] 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.

The electron affinities of the noble gases have not been conclusively measured, so they may or may not have slightly negative EAs. Atoms whose anions are 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.

Contents 1 Values for the elements 2 Periodic trends 3 Molecular electron affinities 4 EA of Surfaces 5 See also 6 References 7 External links

[edit] Values for the elements

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 324	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

[edit] 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. Electron affinity follows the trend of electronegativity. So F has a higher electron affinity than oxygen and so on.

[edit] 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^[2].

[edit] EA of Surfaces

The electron affinity measured from a material's surface is a function of the bulk material as well as the surface condition. Often negative electron affinity is desired to obtain efficient cathodes that can supply electrons to the vacuum with little energy loss. The observed electron yield as a function of various parameters such as bias voltage or illumination conditions can be used to describe these structures with band diagrams in which EA is one parameter. For one illustration of the apparent effect of surface termination on electron emission, see Figure 3 in Marchywka Effect.

[edit] See also

• Electron–capture mass spectrometry
• Koopmans' theorem
• One-electron reduction
• Ionization potential
• Electronegativity
• Valence electron
• Work function
• Marchywka Effect
• Vacuum level
• Ionization energy

[edit] References

1. ^ International Union of Pure and Applied Chemistry. "Electron affinity". Compendium of Chemical Terminology Internet edition.
2. ^ 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

• Tro, Nivaldo J. (2008). Chemistry: A Molecular Approach (2nd Edn.). New Jersey: Pearson Prentice Hall. ISBN 0-13-100065-9. pp. 348–349.

[edit] External links

• Electron affinity, definition from the IUPAC Gold Book