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| In [[chemistry]] and [[atomic physics]], the '''electron affinity''' of an [[atom]] or [[molecule]] is defined as the energy ''change'' when an electron is added to a neutral atom or molecule to form a negative ion.<ref name="Compendiumof">{{GoldBookRef |title=Electron affinity |file=E01977}}</ref>
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| ::X + e<sup>−</sup> → X<sup>−</sup> + energy
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| In [[solid state physics]], the electron affinity for a surface is defined somewhat differently (see below).
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| ==Measurement and use of electron affinity==
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| This property is measured for atoms and molecules in the gaseous state only, since in the solid or liquid states their [[energy level]]s would be changed by contact with other atoms or molecules. A list of the electron affinities was used by [[Robert S. Mulliken]] to develop an [[electronegativity]] scale for atoms, equal to the average of the electron affinity and [[ionization potential]].<ref>Robert S. Mulliken, [[Journal of Chemical Physics]], '''1934''', ''2'', 782.</ref><ref>Modern Physical Organic Chemistry, Eric V. Anslyn and Dennis A. Dougherty, University Science Books, 2006, ISBN 978-1-891389-31-3</ref> Other theoretical concepts that use electron affinity include electronic chemical potential and [[chemical hardness]]. Another example, a molecule or atom that has a more positive value of electron affinity than another is often called an [[electron acceptor]] and the less positive an [[electron donor]]. Together they may undergo [[Intervalence charge transfer|charge-transfer]] reactions.
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| ===Sign convention===
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| To use electron affinities properly, it is essential to keep track of sign. For any reaction that ''releases'' energy, the ''change'' Δ''E'' in [[total energy]] has a negative value and the reaction is called an [[Exothermic reaction|exothermic process]]. Electron capture for almost all non-[[noble gas]] atoms involves the release of energy<ref>Chemical Principles the Quest for Insight, Peter Atkins and Loretta Jones, Freeman, New York, 2010 ISBN 978-1-4292-1955-6</ref> and thus are exothermic. The positive values that are listed in tables of ''E''<sub>ea</sub> are amounts or magnitudes. It is the word, ''released'' within the definition ''energy released'' that supplies the negative sign. Confusion arises in mistaking ''E''<sub>ea</sub> for a change in energy, Δ''E'', in which case the positive values listed in tables would be for an endo- not exo-thermic process. The relation between the two is ''E''<sub>ea</sub> = −Δ''E''(attach).
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| <br>However, if the value assigned to ''E''<sub>ea</sub> is negative, the negative sign implies a reversal of direction, and energy is ''required'' to attach an electron. In this case, the electron capture is an [[endothermic]] process and the relationship, ''E''<sub>ea</sub> = −Δ''E''(attach) is still valid. Negative values typically arise for the capture of a second electron, but also for the nitrogen atom.
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| The usual expression for calculating ''E''<sub>ea</sub> when an electron is attached is
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| ::{{math|size=120%|1=''E''<sub>ea</sub> = (''E''<sub>initial</sub> − ''E''<sub>final</sub>)<sub>attach</sub> = −Δ''E''(attach)}}
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| This expression does follow the convention Δ''X'' = ''X''(final) − ''X''(initial) since −Δ''E'' = −(''E''(final) − ''E''(initial)) = ''E''(initial) − ''E''(final).
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| Equivalently, electron affinity can also be defined as the amount of energy ''required'' to detach an electron from a [[Electric charge|singly charged]] [[ion|negative ion]],<ref name="Compendiumof" /> i.e. the energy change for the process
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| ::X<sup>−</sup> → X + e<sup>−</sup>
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| If the same table is employed for the forward and reverse reactions, ''without switching signs'', care must be taken to apply the correct definition to the corresponding direction, attachment (release) or detachment (require). Since almost all detachments ''(require +)'' an amount of energy listed on the table, those detachment reactions are endothermic, or Δ''E''(detach) > 0.
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| ::{{bigmath|''E''<sub>ea</sub> = (''E''<sub>final</sub> − ''E''<sub>initial</sub>)<sub>detach</sub> = Δ''E''(detach) = −Δ''E''(attach)}}
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| ==Electron affinities of the elements==
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| [[File:Electron affinity of the elements.svg|thumb|right|200px|EA vs Atomic Number]]
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| {{Main|Electron affinity (data page)}}
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| Although ''E''<sub>ea</sub> varies greatly across the periodic table, some patterns emerge. Generally, [[nonmetals]] have more positive ''E''<sub>ea</sub> than [[metal]]s. Atoms whose anions are more stable than neutral atoms have a greater ''E''<sub>ea</sub>. [[Chlorine]] most strongly attracts extra electrons; [[mercury (element)|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.
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| ''E''<sub>ea</sub> generally increases across a period (row) in the periodic table. This is caused by the filling of the valence shell of the atom; a [[halogen|Group 17]] atom releases more energy than a [[Group 1 element|Group 1]] atom on gaining an electron because it obtains a filled [[electron shell|valence shell]] and therefore is more stable.
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| A trend of decreasing ''E''<sub>ea</sub> going down the groups in the periodic table would be expected. The additional electron will be entering an orbital farther away from the nucleus. Since this electron is farther from the nucleus it is less attracted to the nucleus and would release less energy when added. However, a clear counterexample to this trend can be found in [[Group 2 element|Group 2]], and this trend only applies to Group 1 atoms.
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| Electron affinity follows the trend of electronegativity. [[Fluorine]] (F) has a higher electron affinity than oxygen and so on.
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| The following data are quoted in [[Joule per mole|kJ/mol]]. Elements marked with an asterisk are expected to have electron affinities close to zero on quantum mechanical grounds.
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| <center>{{periodic table (electron affinities)}}</center>
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| ==Molecular electron affinities==
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| The electron affinity of molecules is a complicated function of their electronic structure.
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| 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]].<ref>''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 [http://dx.doi.org/10.1039/b515843e Abstract]</ref>
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| =="Electron affinity" as defined in solid state physics ==
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| [[File:Semiconductor vacuum junction.svg|thumb|[[Band diagram]] of semiconductor-vacuum interface showing electron affinity ''E''<sub>EA</sub>, defined as the difference between near-surface vacuum energy ''E''<sub>vac</sub>, and near-surface [[conduction band]] edge ''E''<sub>C</sub>. Also shown: [[Fermi level]] ''E''<sub>F</sub>, [[valence band]] edge ''E''<sub>V</sub>, [[work function]] ''W''.]]
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| In the field of [[solid state physics]], the electron affinity is defined differently from in chemistry.
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| For a semiconductor-vacuum interface (that is, the surface of a semiconductor), electron affinity, typically denoted by ''E''<sub>EA</sub> or ''χ'', is defined instead as the energy obtained by moving an electron from the vacuum just outside the semiconductor to the bottom of the [[conduction band]] just inside the semiconductor:<ref>http://academic.brooklyn.cuny.edu/physics/tung/Schottky/surface.htm</ref>
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| :<math>E_{\rm EA} \equiv E_{\rm vac} - E_{\rm C}</math>
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| In an intrinsic semiconductor at [[absolute zero]], this concept is analogous to the chemistry definition of electron affinity, since an added electron will go to the bottom of the conduction band. At nonzero temperature, and for other materials (metals, semimetals, heavily doped semiconductors), the analogy does not hold since an added electron will instead go to the [[Fermi level]] on average. In any case, the electron affinity of a solid substance is very different from the chemistry electron affinity of that substance in gas phase. For example, a silicon crystal surface has electron affinity 4.05 eV, whereas an isolated silicon atom has electron affinity 1.39 eV.
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| The electron affinity is a similar concept to the [[work function]], but distinct. The work function is the [[thermodynamic work]] that can be obtained by reversibly, isothermally moving an electron from the vacuum to the material; this thermodynamic electron goes to the [[Fermi level]] on average, not the conduction band edge: <math> W = E_{\rm vac} - E_{\rm F}</math>.
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| While the [[work function]] of a semiconductor can be changed by [[doping (semiconductor)|doping]], the electron affinity ideally does not change with doping and so it is closer to being a material constant.
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| However, the electron affinity does depend on the surface termination (crystal face, surface chemistry, etc.).
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| In certain circumstances, the electron affinity may become negative.<ref>{{cite doi|10.1103/PhysRevB.20.624}}</ref>
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| Often negative electron affinity is desired to obtain efficient [[cathode]]s 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 diagram]]s in which the electron affinity is one parameter. For one illustration of the apparent effect of surface termination on electron emission, see Figure 3 in [[Marchywka Effect]].
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| In semiconductor physics, the primary use of the electron affinity is not actually in the analysis of semiconductor-vacuum junctions, but rather in heuristic [[electron affinity rule]]s for estimating the [[band bending]] that occurs at the interface of two materials.
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| ==See also==
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| * [[Ionization energy]] — a closely related concept describing the energy required to ''remove'' an electron from a neutral atom or molecule
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| * [[One-electron reduction]]
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| * [[Electron-capture mass spectrometry]]
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| * [[Electronegativity]]
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| * [[Valence electron]]
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| * [[Vacuum level]]
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| * [[Electron donor]]
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| ==References==
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| {{reflist}}
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| *Tro, Nivaldo J. (2008). ''Chemistry: A Molecular Approach'' (2nd Edn.). New Jersey: [[Pearson Prentice Hall]]. ISBN 0-13-100065-9. pp. 348–349.
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| ==External links==
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| * [http://www.iupac.org/goldbook/E01977.pdf Electron affinity], definition from the [[IUPAC]] [[Gold Book]]
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| {{periodic tables footer}}
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| {{DEFAULTSORT:Electron Affinity}}
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| [[Category:Chemical bonding]]
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| [[Category:Chemical properties]]
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| [[Category:Molecular physics]]
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