Electron affinity
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
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
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
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
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
Although Eea varies greatly across the periodic table, some patterns emerge. Generally,
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
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
The following data are quoted in kJ/mol.
Group →
|
1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | |||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
↓ Period
|
|||||||||||||||||||||
1 | H 73 | He(−50) | |||||||||||||||||||
2 | Li60 | Be(−50) | B 27 | C 122 | N −7 | O 141 | F 328 | Ne(−120) | |||||||||||||
3 | Na53 | Mg(−40) | Al42 | Si134 | P 72 | S 200 | Cl349 | Ar(−96) | |||||||||||||
4 | K 48 | Ca2 | Sc18 | Ti7 | V 51 | Cr65 | Mn(−50) | Fe15 | Co64 | Ni112 | Cu119 | Zn(−60) | Ga29 | Ge119 | As78 | Se195 | Br325 | Kr(−96) | |||
5 | Rb47 | Sr5 | Y 30 | Zr42 | Nb89 | Mo72 | Tc(53) | Ru(101) | Rh110 | Pd54 | Ag126 | Cd(−70) | In37 | Sn107 | Sb101 | Te190 | I 295 | Xe(−80) | |||
6 | Cs46 | Ba14 | Lu23 | Hf17 | Ta31 | W 79 | 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 in kJ/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 |
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
In the field of
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
In semiconductor physics, the primary use of the electron affinity is not actually in the analysis of semiconductor–vacuum surfaces, but rather in heuristic
In certain circumstances, the electron affinity may become negative.
See also
- Electron-capture mass spectrometry
- Electronegativity
- Electron donor
- Ionization energy — a closely related concept describing the energy required to remove an electron from a neutral atom or molecule
- One-electron reduction
- Valence electron
- Vacuum level
References
- ^
- Journal of Chemical Physics, 1934, 2, 782.
- ISBN 978-1-891389-31-3
- ISBN 978-1-4292-1955-6
- Chemical Communications, 2006, 758–760 Abstract
- ^ Tung, Raymond T. "Free Surfaces of Semiconductors". Brooklyn College.
- .
- Tro, Nivaldo J. (2008). Chemistry: A Molecular Approach (2nd Edn.). New Jersey: ISBN 0-13-100065-9. pp. 348–349.
External links
- Electron affinity, definition from the Gold Book