Theelectron affinity (Eea) of anatom ormolecule 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.
This differs by sign from the energy change ofelectron capture ionization.[1] The electron affinity is positive when energy is released on electron capture.
Insolid state physics, the electron affinity for a surface is defined somewhat differently (see below).
This property is used to measure atoms and molecules in the gaseous state only, since in a solid or liquid state theirenergy levels would be changed by contact with other atoms or molecules.
A list of the electron affinities was used byRobert S. Mulliken to develop anelectronegativity scale for atoms, equal to the average of the electronsaffinity andionization potential.[2][3] Other theoretical concepts that use electron affinity include electronic chemical potential andchemical hardness. Another example, a molecule or atom that has a more positive value of electron affinity than another is often called anelectron acceptor and the less positive anelectron donor. Together they may undergocharge-transfer reactions.
To use electron affinities properly, it is essential to keep track of sign. For any reaction thatreleases energy, thechange ΔE intotal energy has a negative value and the reaction is called anexothermic 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 ofEea are amounts or magnitudes. It is the word "released" within the definition "energy released" that supplies the negative sign to ΔE. Confusion arises in mistakingEea 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 isEea = −ΔE(attach).
However, if the value assigned toEea is negative, the negative sign implies a reversal of direction, and energy isrequired to attach an electron. In this case, the electron capture is anendothermic 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 calculatingEea when an electron is attached is
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 energyrequired to detach an electron from the atom while it holds asingle-excess-electron thus making the atom anegative ion,[1] i.e. the energy change for the process
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.
AlthoughEea varies greatly across theperiodic table, some patterns emerge. Generally,nonmetals have more positiveEea thanmetals. Atoms whose anions are more stable than neutral atoms have a greaterEea.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; agroup 17 atom releases more energy than agroup 1 atom on gaining an electron because it obtains a filledvalence 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 doesnot decrease when progressing down most columns of the periodic table. For example,Eea actually increases consistently on descending the column for thegroup 2 data. Thus, electron affinity follows the same "left-right" trend as electronegativity, but not the "up-down" trend.
The following data are quoted inkJ/mol.
| Group → | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | |||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| ↓ Period | |||||||||||||||||||||
| 1 | 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(<0) | Nh(67) | Fl(<0) | Mc(35) | Lv(75) | Ts(166) | Og(8) | ||
| La54 | Ce55 | Pr11 | Nd9 | Pm(12) | Sm(16) | Eu11 | Gd(13) | Tb13 | Dy1 | 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) | |||||||
| Legend | |||||||||||||||||||||
| Values are inkJ/mol, rounded | |||||||||||||||||||||
| For the equivalent in eV, see:Electron affinity (data page) | |||||||||||||||||||||
| Parentheses or Round brackets() denote predictions | |||||||||||||||||||||
Primordial From decay Synthetic Border shows natural occurrence of the element | |||||||||||||||||||||
The electron affinity of molecules is a complicated function of their electronic structure.For instance the electron affinity forbenzene is negative, as is that ofnaphthalene, while those ofanthracene,phenanthrene andpyrene are positive.In silico experiments show that the electron affinity ofhexacyanobenzene surpasses that offullerene.[5]
In the field ofsolid 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 byEEA orχ, is defined as the energy obtained by moving an electron from the vacuum just outside the semiconductor to the bottom of theconduction band just inside the semiconductor:[6]
In an intrinsic semiconductor atabsolute 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, itswork function. The work function is thethermodynamic work that can be obtained by reversibly and isothermally removing an electron from the material to vacuum; this thermodynamic electron goes to theFermi level on average, not the conduction band edge:. While the work function of a semiconductor can be changed bydoping, 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 heuristicelectron affinity rules for estimating theband bending that occurs at the interface of two materials, in particularmetal–semiconductor junctions and semiconductorheterojunctions.
In certain circumstances, the electron affinity may become negative.[7] Often negative electron affinity is desired to obtain efficientcathodes 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 withband 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 inMarchywka Effect.
