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Electron affinity

April 09, 2023

The electron affinity (Eea) of an atom or molecule is defined as the amount of energy released when an electron attaches to a neutral atom or molecule in the gaseous state to form an anion.

X(g) + e− → X−(g) + energy

This differs by sign from the energy change of electron capture ionization.[1] The electron affinity is positive when energy is released on electron capture.

In solid state physics, the electron affinity for a surface is defined somewhat differently (see below).

Measurement and use of electron affinity

This property is used to measure atoms and molecules in the gaseous state only, since in a solid or liquid state their energy levels 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 electrons affinity and ionization potential.[2][3] 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 charge-transfer reactions.

Sign convention

To use electron affinities properly, it is essential to keep track of sign. For any reaction that releases energy, the change ΔE in total energy has a negative value and the reaction is called an exothermic process. Electron capture for almost all non-noble gas atoms involves the release of energy[4] and thus is exothermic. The positive values that are listed in tables of Eea are amounts or magnitudes. It is the word "released" within the definition "energy released" that supplies the negative sign to ΔE. Confusion arises in mistaking Eea for a change in energy, ΔE, in which case the positive values listed in tables would be for an endo- not exo-thermic process. The relation between the two is Eea = −ΔE(attach).

However, if the value assigned to Eea 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, Eea = −ΔE(attach) is still valid. Negative values typically arise for the capture of a second electron, but also for the nitrogen atom.

The usual expression for calculating Eea when an electron is attached is

Eea = (Einitial − Efinal)attach = −ΔE(attach)

This expression does follow the convention ΔX = X(final) − X(initial) since −ΔE = −(E(final) − E(initial)) = E(initial) − E(final).

Equivalently, electron affinity can also be defined as the amount of energy required to detach an electron from the atom while it holds a single-excess-electron thus making the atom a negative ion,[1] i.e. the energy change for the process

X− → X + e−

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 ΔE(detach) > 0.

Eea = (Efinal − Einitial)detach = ΔE(detach) = −ΔE(attach).

Electron affinities of the elements

 
Electron affinity (Eea) vs atomic number (Z). Note the sign convention explanation in the previous section.
Main article: Electron affinity (data page)

Although Eea varies greatly across the periodic table, some patterns emerge. Generally, nonmetals have more positive Eea than metals. Atoms whose anions are more stable than neutral atoms have a greater Eea. Chlorine most strongly attracts extra electrons; neon 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.

Eea generally increases across a period (row) in the periodic table prior to reaching group 18. This is caused by the filling of the valence shell of the atom; a group 17 atom releases more energy than a group 1 atom on gaining an electron because it obtains a filled valence shell and therefore is more stable. In group 18, the valence shell is full, meaning that added electrons are unstable, tending to be ejected very quickly.

Counterintuitively, Eea does not decrease when progressing down most columns of the periodic table. For example, Eea actually increases consistently on descending the column for the group 2 data. Thus, electron affinity follows the same "left-right" trend as electronegativity, but not the "up-down" trend.

The following data are quoted in kJ/mol.

Molecular electron affinities

The electron affinity of molecules is a complicated function of their electronic structure. 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.[5]

"Electron affinity" as defined in solid state physics

 
Band diagram of semiconductor-vacuum interface showing electron affinity EEA, defined as the difference between near-surface vacuum energy Evac, and near-surface conduction band edge EC. Also shown: Fermi level EF, valence band edge EV, work function W.

In the field of solid state physics, the electron affinity is defined differently than in chemistry and atomic physics. For a semiconductor-vacuum interface (that is, the surface of a semiconductor), electron affinity, typically denoted by EEA or χ, is defined 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:[6]

 

In an intrinsic semiconductor at absolute zero, this concept is functionally analogous to the chemistry definition of electron affinity, since an added electron will spontaneously 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 value of the electron affinity of a solid substance is very different from the chemistry and atomic physics electron affinity value for an atom of the same 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.

The electron affinity of a surface is closely related to, but distinct from, its work function. The work function is the thermodynamic work that can be obtained by reversibly and isothermally removing an electron from the material to vacuum; this thermodynamic electron goes to the Fermi level on average, not the conduction band edge:  . While the work function of a semiconductor can be changed by doping, the electron affinity ideally does not change with doping and so it is closer to being a material constant. However, like work function the electron affinity does depend on the surface termination (crystal face, surface chemistry, etc.) and is strictly a surface property.

In semiconductor physics, the primary use of the electron affinity is not actually in the analysis of semiconductor–vacuum surfaces, but rather in heuristic electron affinity rules for estimating the band bending that occurs at the interface of two materials, in particular metal–semiconductor junctions and semiconductor heterojunctions.

In certain circumstances, the electron affinity may become negative.[7] 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 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.

See also

References

  1. ^ a b IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "Electron affinity". doi:10.1351/goldbook.E01977
  2. ^ Robert S. Mulliken, Journal of Chemical Physics, 1934, 2, 782.
  3. ^ Modern Physical Organic Chemistry, Eric V. Anslyn and Dennis A. Dougherty, University Science Books, 2006, ISBN 978-1-891389-31-3
  4. ^ Chemical Principles the Quest for Insight, Peter Atkins and Loretta Jones, Freeman, New York, 2010 ISBN 978-1-4292-1955-6
  5. ^ 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
  6. ^ Tung, Raymond T. "Free Surfaces of Semiconductors". Brooklyn College.
  7. ^ Himpsel, F.; Knapp, J.; Vanvechten, J.; Eastman, D. (1979). "Quantum photoyield of diamond(111)—A stable negative-affinity emitter". Physical Review B. 20 (2): 624. Bibcode:1979PhRvB..20..624H. doi:10.1103/PhysRevB.20.624.

External links

April 09, 2023
electron, affinity, electron, affinity, atom, molecule, defined, amount, energy, released, when, electron, attaches, neutral, atom, molecule, gaseous, state, form, anion, energy, this, differs, sign, from, energy, change, electron, capture, ionization, electro. The electron affinity Eea of an atom or molecule is defined as the amount of energy released when an electron attaches to a neutral atom or molecule in the gaseous state to form an anion X g e X g energy dd This differs by sign from the energy change of electron capture ionization 1 The electron affinity is positive when energy is released on electron capture In solid state physics the electron affinity for a surface is defined somewhat differently see below Contents 1 Measurement and use of electron affinity 1 1 Sign convention 2 Electron affinities of the elements 3 Molecular electron affinities 4 Electron affinity as defined in solid state physics 5 See also 6 References 7 External linksMeasurement and use of electron affinity EditThis property is used to measure atoms and molecules in the gaseous state only since in a solid or liquid state their energy levels 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 electrons affinity and ionization potential 2 3 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 charge transfer reactions Sign convention Edit To use electron affinities properly it is essential to keep track of sign For any reaction that releases energy the change DE in total energy has a negative value and the reaction is called an exothermic process Electron capture for almost all non noble gas atoms involves the release of energy 4 and thus is exothermic The positive values that are listed in tables of Eea are amounts or magnitudes It is the word released within the definition energy released that supplies the negative sign to DE Confusion arises in mistaking Eea for a change in energy DE in which case the positive values listed in tables would be for an endo not exo thermic process The relation between the two is Eea DE attach However if the value assigned to Eea 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 Eea DE attach is still valid Negative values typically arise for the capture of a second electron but also for the nitrogen atom The usual expression for calculating Eea when an electron is attached is Eea Einitial Efinal attach DE attach This expression does follow the convention DX X final X initial since DE E final E initial E initial E final Equivalently electron affinity can also be defined as the amount of energy required to detach an electron from the atom while it holds a single excess electron thus making the atom a negative ion 1 i e the energy change for the process X X e 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 DE detach gt 0 Eea Efinal Einitial detach DE detach DE attach Electron affinities of the elements Edit Electron affinity Eea vs atomic number Z Note the sign convention explanation in the previous section Main article Electron affinity data page Although Eea varies greatly across the periodic table some patterns emerge Generally nonmetals have more positive Eea than metals Atoms whose anions are more stable than neutral atoms have a greater Eea Chlorine most strongly attracts extra electrons neon 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 Eea generally increases across a period row in the periodic table prior to reaching group 18 This is caused by the filling of the valence shell of the atom a group 17 atom releases more energy than a group 1 atom on gaining an electron because it obtains a filled valence shell and therefore is more stable In group 18 the valence shell is full meaning that added electrons are unstable tending to be ejected very quickly Counterintuitively Eea does not decrease when progressing down most columns of the periodic table For example Eea actually increases consistently on descending the column for the group 2 data Thus electron affinity follows the same left right trend as electronegativity but not the up down trend The following data are quoted in kJ mol vteElectron affinities in the periodic tableGroup 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Period1 H73 He 50 2 Li60 Be 50 B27 C122 N 7 O141 F328 Ne 120 3 Na53 Mg 40 Al42 Si134 P72 S200 Cl349 Ar 96 4 K48 Ca2 Sc18 Ti7 V51 Cr65 Mn 50 Fe15 Co64 Ni112 Cu119 Zn 60 Ga29 Ge119 As78 Se195 Br325 Kr 96 5 Rb47 Sr5 Y30 Zr42 Nb89 Mo72 Tc 53 Ru 101 Rh110 Pd54 Ag126 Cd 70 In37 Sn107 Sb101 Te190 I295 Xe 80 6 Cs46 Ba14 Lu23 Hf17 Ta31 W79 Re6 Os104 Ir151 Pt205 Au223 Hg 50 Tl31 Pb34 Bi91 Po 136 At233 Rn 70 7 Fr 47 Ra 10 Lr 30 Rf Db Sg Bh Hs Mt Ds Rg 151 Cn lt 0 Nh 67 Fl lt 0 Mc 35 Lv 75 Ts 166 Og 8 La54 Ce55 Pr11 Nd9 Pm 12 Sm 16 Eu11 Gd 13 Tb13 Dy gt 34 Ho 33 Er 30 Tm99 Yb 2 Ac 34 Th 113 Pa 53 U 51 Np 46 Pu 48 Am 10 Cm 27 Bk 165 Cf 97 Es 29 Fm 34 Md 94 No 223 LegendValues are in kJ mol roundedFor the equivalent in eV see Electron affinity data page Parentheses or Round brackets denote predictionsPrimordial From decay Synthetic Border shows natural occurrence of the elements block f block d block p blockMolecular electron affinities EditThe electron affinity of molecules is a complicated function of their electronic structure 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 5 Electron affinity as defined in solid state physics Edit Band diagram of semiconductor vacuum interface showing electron affinity EEA defined as the difference between near surface vacuum energy Evac and near surface conduction band edge EC Also shown Fermi level EF valence band edge EV work function W In the field of solid state physics the electron affinity is defined differently than in chemistry and atomic physics For a semiconductor vacuum interface that is the surface of a semiconductor electron affinity typically denoted by EEA or x is defined 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 6 E e a E v a c E C displaystyle E rm ea equiv E rm vac E rm C In an intrinsic semiconductor at absolute zero this concept is functionally analogous to the chemistry definition of electron affinity since an added electron will spontaneously 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 value of the electron affinity of a solid substance is very different from the chemistry and atomic physics electron affinity value for an atom of the same 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 The electron affinity of a surface is closely related to but distinct from its work function The work function is the thermodynamic work that can be obtained by reversibly and isothermally removing an electron from the material to vacuum this thermodynamic electron goes to the Fermi level on average not the conduction band edge W E v a c E F displaystyle W E rm vac E rm F While the work function of a semiconductor can be changed by doping the electron affinity ideally does not change with doping and so it is closer to being a material constant However like work function the electron affinity does depend on the surface termination crystal face surface chemistry etc and is strictly a surface property In semiconductor physics the primary use of the electron affinity is not actually in the analysis of semiconductor vacuum surfaces but rather in heuristic electron affinity rules for estimating the band bending that occurs at the interface of two materials in particular metal semiconductor junctions and semiconductor heterojunctions In certain circumstances the electron affinity may become negative 7 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 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 See also EditIonization energy a closely related concept describing the energy required to remove an electron from a neutral atom or molecule One electron reduction Electron capture mass spectrometry Electronegativity Valence electron Vacuum level Electron donorReferences Edit a b IUPAC Compendium of Chemical Terminology 2nd ed the Gold Book 1997 Online corrected version 2006 Electron affinity doi 10 1351 goldbook E01977 Robert S Mulliken Journal of Chemical Physics 1934 2 782 Modern Physical Organic Chemistry Eric V Anslyn and Dennis A Dougherty University Science Books 2006 ISBN 978 1 891389 31 3 Chemical Principles the Quest for Insight Peter Atkins and Loretta Jones Freeman New York 2010 ISBN 978 1 4292 1955 6 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 Tung Raymond T Free Surfaces of Semiconductors Brooklyn College Himpsel F Knapp J Vanvechten J Eastman D 1979 Quantum photoyield of diamond 111 A stable negative affinity emitter Physical Review B 20 2 624 Bibcode 1979PhRvB 20 624H doi 10 1103 PhysRevB 20 624 Tro Nivaldo J 2008 Chemistry A Molecular Approach 2nd Edn New Jersey Pearson Prentice Hall ISBN 0 13 100065 9 pp 348 349 External links EditElectron affinity definition from the IUPAC Gold Book Retrieved from https en wikipedia org w index php title Electron affinity amp oldid 1146963628, wikipedia, wiki, book, books, library,

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