Electron affinity

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Electron affinity EEA in a band diagram for solids. EVAC, EC, EF, EV stand for vacuum energy, conduction band minimum, fermi energy and the valence band maximum, respectively. In atoms the lowest unoccupied and highest occupied atomic orbitals correspond approximately to EC and EV, respectively.

The electron affinity of an atom or molecule is defined as the amount of energy released when an electron is added to a neutral atom or molecule to form a negative ion.[1]

X + e → X

This property is measured for atoms and molecules in the gaseous state only, since in the solid or liquid states 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 electron 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.

In solids, the electron affinity is the energy difference between the vacuum energy and the conduction band minimum.[4]

To use electron affinities properly, it is essential to keep track of sign. For any reaction that releases energy, the change in energy, ΔE, 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[5] and thus are 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. 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 a singly charged 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, 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; 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.

Eea 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 and therefore is more stable.

A trend of decreasing Eea 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 2A, and this trend only applies to group 1A atoms. Electron affinity follows the trend of electronegativity. Fluorine (F) has a higher electron affinity than oxygen and so on.

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.

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

Lanthanides
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
 

Actinides
Rf
 
Db
 
Sg
 
Bh
 
Hs
 
Mt
 
Ds
 
Rg
 
Cn
 
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
 
  1. REDIRECT Template:Element color legend/metal–nonmetal range
Atomic number colors show state of matter at standard conditions: (0 °C and 1 atm)
Solids Liquids Gases Unknown
Borders show natural occurrence:
Primordial From decay Synthetic

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.[6]

Electron affinity 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 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. 1.0 1.1 Nic, M.; Jirat, J.; Kosata, B., eds. (2006–). "Electron affinity". IUPAC Compendium of Chemical Terminology (Online ed.). doi:10.1351/goldbook.E01977. ISBN 0-9678550-9-8.  Check date values in: |date= (help)
  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. Festkörperphysik. Einführung in die Grundlagen, Harald Ibach, Hans Lüth, Springer, Berlin, 1999, 5.th edition
  5. Chemical Principles the Quest for Insight, Peter Atkins and Loretta Jones, Freeman, New York, 2010 ISBN 978-1-4292-1955-6
  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
  • 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