Electron affinity
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2008-07-11T15:37:18Z
Mild Bill Hiccup
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The '''electron affinity''', ''E''<sub>ea</sub>, of an [[atom]] or [[molecule]] is the energy required to detach an electron from a [[Electric charge|singly charged negative]] [[ion]], i.e., the energy change for the process
::X<sup>-</sup> → X + e<sup>−</sup>
An equivalent definition is the energy released (''E''<sub>initial</sub> − ''E''<sub>final</sub>) when an electron is attached to a neutral atom or molecule. It should be noted that the [[sign convention]] for ''E''<sub>ea</sub> is the opposite to most [[thermodynamic]] quantities: a positive electron affinity indicates that energy is ''released'' on going from atom to [[anion]].
All elements whose EA have been measured using modern methods have a positive electron affinity, but older texts mistakenly report that some elements such as alkaline earth metals have negative ''E''<sub>ea</sub>, meaning they would repel electrons.{{Fact|date=May 2008}} This is not recognized by modern chemists. The electron affinities of the noble gases have not been conclusively measured, so they may or may not have slightly negative EAs. Atoms whose anions are relatively more stable than neutral atoms have a greater ''E''<sub>ea</sub>. [[Fluorine]] most strongly attracts extra electrons; [[mercury (element)|mercury]] most weakly attracts an extra electron. ''E''<sub>ea</sub> of noble gases are close to 0.
Although ''E''<sub>ea</sub> vary in a chaotic manner across the table, some patterns emerge. Generally, [[nonmetals]] have more positive ''E''<sub>ea</sub> than [[metal]]s.
==Values for the elements==
{{main|Electron affinity (data page)}}
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. Elements marked with a dotted box are synthetically made elements—elements not found naturally in the environment.
<center>
{| border="0" cellpadding="0" cellspacing="2" style="width:80%; {{{1|}}}"
|-
! width="1.0%" | <small>[[Periodic table group|Group]] →</small>
! width="5.5%" | [[Alkali metal|1]]
! width="5.5%" | [[Alkaline earth metal|2]]
! width="5.5%" | [[Group 3 element|3]]
! width="5.5%" | [[Group 7 element|4]]
! width="5.5%" | [[Group 5 element|5]]
! width="5.5%" | [[Group 6 element|6]]
! width="5.5%" | [[Group 7 element|7]]
! width="5.5%" | [[Group 8 element|8]]
! width="5.5%" | [[Group 9 element|9]]
! width="5.5%" | [[Group 10 element|10]]
! width="5.5%" | [[Group 11 element|11]] <!-- also [[Coinage metal|11]] -->
! width="5.5%" | [[Group 12 element|12]]
! width="5.5%" | [[Boron group|13]]
! width="5.5%" | [[Carbon group|14]]
! width="5.5%" | [[Nitrogen group|15]] <!-- also [[Pnictogen|15]] -->
! width="5.5%" | [[Chalcogen|16]]
! width="5.5%" | [[Halogen|17]]
! width="5.5%" | [[Noble gas|18]]
|-
! <small>↓ [[Periodic table period|Period]]</small>
| colspan="19"|<br />
|-
! [[Period 1 element|1]]
| {{element cell property|H |hydrogen |73 |Nonmetals|Primordial}}
| colspan="16"|<br />
| {{element cell property|He|helium |* |Noble gases|Primordial}}
|-
! [[Period 2 element|2]]
| {{element cell property|Li|lithium |60 |Alkali metals|Primordial}}
| {{element cell property|Be|beryllium |* |Alkaline earth metals|Primordial}}
| colspan="10"|<br />
| {{element cell property|B |boron |27 |Metalloids|Primordial}}
| {{element cell property|C |carbon |122 |Nonmetals|Primordial}}
| {{element cell property|N |nitrogen |* |Nonmetals|Primordial}}
| {{element cell property|O |oxygen |141 |Nonmetals|Primordial}}
| {{element cell property|F |fluorine |328 |Halogens|Primordial}}
| {{element cell property|Ne|neon |* |Noble gases|Primordial}}
|-
! [[Period 3 element|3]]
| {{element cell property|Na|sodium |53 |Alkali metals|Primordial}}
| {{element cell property|Mg|magnesium |* |Alkaline earth metals|Primordial}}
| colspan="10"|<br />
| {{element cell property|Al|aluminium |42 |Poor metals|Primordial}}
| {{element cell property|Si|silicon |134 |Metalloids|Primordial}}
| {{element cell property|P |phosphorus|72 |Nonmetals|Primordial}}
| {{element cell property|S |sulfur |200 |Nonmetals|Primordial}}
| {{element cell property|Cl|chlorine |''349'' |Halogens|Primordial}}
| {{element cell property|Ar|argon |* |Noble gases|Primordial}}
|-
! [[Period 4 element|4]]
| {{element cell property|K |potassium |48 |Alkali metals|Primordial}}
| {{element cell property|Ca|calcium |2 |Alkaline earth metals|Primordial}}
| {{element cell property|Sc|scandium |18|Transition metals|Primordial}}
| {{element cell property|Ti|titanium |8 |Transition metals|Primordial}}
| {{element cell property|V |vanadium |51 |Transition metals|Primordial}}
| {{element cell property|Cr|chromium |65 |Transition metals|Primordial}}
| {{element cell property|Mn|manganese |* |Transition metals|Primordial}}
| {{element cell property|Fe|iron |15 |Transition metals|Primordial}}
| {{element cell property|Co|cobalt |64 |Transition metals|Primordial}}
| {{element cell property|Ni|nickel |112 |Transition metals|Primordial}}
| {{element cell property|Cu|copper |119 |Transition metals|Primordial}}
| {{element cell property|Zn|zinc |* |Transition metals|Primordial}}
| {{element cell property|Ga|gallium |41 |Poor metals|Primordial}}
| {{element cell property|Ge|germanium |119 |Metalloids|Primordial}}
| {{element cell property|As|arsenic |79 |Metalloids|Primordial}}
| {{element cell property|Se|selenium |195 |Nonmetals|Primordial}}
| {{element cell property|Br|bromine |324 |Halogens|Primordial}}
| {{element cell property|Kr|krypton |* |Noble gases|Primordial}}
|-
! [[Period 5 element|5]]
| {{element cell property|Rb|rubidium |47 |Alkali metals|Primordial}}
| {{element cell property|Sr|strontium |5 |Alkaline earth metals|Primordial}}
| {{element cell property|Y |yttrium |30 |Transition metals|Primordial}}
| {{element cell property|Zr|zirconium |41 |Transition metals|Primordial}}
| {{element cell property|Nb|niobium |86 |Transition metals|Primordial}}
| {{element cell property|Mo|molybdenum|72 |Transition metals|Primordial}}
| {{element cell property|Tc|technetium|* |Transition metals|Synthetic}}
| {{element cell property|Ru|ruthenium |101|Transition metals|Primordial}}
| {{element cell property|Rh|rhodium |110|Transition metals|Primordial}}
| {{element cell property|Pd|palladium |54|Transition metals|Primordial}}
| {{element cell property|Ag|silver |126|Transition metals|Primordial}}
| {{element cell property|Cd|cadmium |*|Transition metals|Primordial}}
| {{element cell property|In|indium |39|Poor metals|Primordial}}
| {{element cell property|Sn|tin |107|Poor metals|Primordial}}
| {{element cell property|Sb|antimony |101|Metalloids|Primordial}}
| {{element cell property|Te|tellurium |190|Metalloids|Primordial}}
| {{element cell property|I |iodine |''295''|Halogens|Primordial}}
| {{element cell property|Xe|xenon |*|Noble gases|Primordial}}
|-
! [[Period 6 element|6]]
| {{element cell property|Cs|caesium |46|Alkali metals|Primordial}}
| {{element cell property|Ba|barium |14|Alkaline earth metals|Primordial}}
| {{element cell | * |Lanthanides| | |Solid|Lanthanides|Undiscovered}}
| {{element cell property|Hf|hafnium | |Transition metals|Primordial}}
| {{element cell property|Ta|tantalum |31|Transition metals|Primordial}}
| {{element cell property|W |tungsten |79|Transition metals|Primordial}}
| {{element cell property|Re|rhenium |*|Transition metals|Primordial}}
| {{element cell property|Os|osmium |104|Transition metals|Primordial}}
| {{element cell property|Ir|iridium |150|Transition metals|Primordial}}
| {{element cell property|Pt|platinum |205|Transition metals|Primordial}}
| {{element cell property|Au|gold |223|Transition metals|Primordial}}
| {{element cell property|Hg|mercury |*|Transition metals|Primordial|mercury (element)}}
| {{element cell property|Tl|thallium |36|Poor metals|Primordial}}
| {{element cell property|Pb|lead |35 |Poor metals|Primordial}}
| {{element cell property|Bi|bismuth |91|Poor metals|Primordial}}
| {{element cell property|Po|polonium | |Metalloids|Natural radio}}
| {{element cell property|At|astatine | |Halogens|Natural radio}}
| {{element cell property|Rn|radon |* |Noble gases|Natural radio}}
|-
! [[Period 7 element|7]]
| {{element cell property|Fr|francium | |Alkali metals|Natural radio}}
| {{element cell property|Ra|radium | |Alkaline earth metals|Natural radio}}
| {{element cell | ** |Actinides| | |Solid|Actinides|Undiscovered}}
| {{element cell property|Rf|rutherfordium| |Transition metals|Synthetic}}
| {{element cell property|Db|dubnium | |Transition metals|Synthetic}}
| {{element cell property|Sg|seaborgium | |Transition metals|Synthetic}}
| {{element cell property|Bh|bohrium | |Transition metals|Synthetic}}
| {{element cell property|Hs|hassium | |Transition metals|Synthetic}}
| {{element cell property|Mt|meitnerium | |Transition metals|Synthetic}}
| {{element cell property|Ds|darmstadtium | |Transition metals|Synthetic}}
| {{element cell property|Rg|roentgenium | |Transition metals|Synthetic}}
| {{element cell property|Uub|ununbium | |Transition metals|Synthetic}}
| {{element cell property|Uut|ununtrium | |Poor metals|Synthetic}}
| {{element cell property|Uuq|ununquadium | |Poor metals|Synthetic}}
| {{element cell property|Uup|ununpentium | |Poor metals|Synthetic}}
| {{element cell property|Uuh|ununhexium | |Poor metals|Synthetic}}
| {{element cell property|Uus|ununseptium | |Halogens|Undiscovered|ununseptium|#fcfece}}
| {{element cell property|Uuo|ununoctium | |Noble gases|Synthetic}}
|-
| colspan="21"|<br />
|-
| colspan="4" style="text-align:right"|* '''[[Lanthanide]]s'''
| {{element cell property|La|lanthanum |45|Lanthanides|Primordial}}
| {{element cell property|Ce|cerium |92|Lanthanides|Primordial}}
| {{element cell property|Pr|praseodymium| |Lanthanides|Primordial}}
| {{element cell property|Nd|neodymium | |Lanthanides|Primordial}}
| {{element cell property|Pm|promethium | |Lanthanides|Synthetic}}
| {{element cell property|Sm|samarium | |Lanthanides|Primordial}}
| {{element cell property|Eu|europium | |Lanthanides|Primordial}}
| {{element cell property|Gd|gadolinium | |Lanthanides|Primordial}}
| {{element cell property|Tb|terbium | |Lanthanides|Primordial}}
| {{element cell property|Dy|dysprosium | |Lanthanides|Primordial}}
| {{element cell property|Ho|holmium | |Lanthanides|Primordial}}
| {{element cell property|Er|erbium | |Lanthanides|Primordial}}
| {{element cell property|Tm|thulium |99|Lanthanides|Primordial}}
| {{element cell property|Yb|ytterbium | |Lanthanides|Primordial}}
| {{element cell property|Lu|lutetium |33|Lanthanides|Primordial}}
|-
| colspan="4" style="text-align:right"|** '''[[Actinide]]s'''
| {{element cell property|Ac|actinium | |Actinides|Natural radio}}
| {{element cell property|Th|thorium | |Actinides|Primordial}}
| {{element cell property|Pa|protactinium| |Actinides|Natural radio}}
| {{element cell property|U |uranium | |Actinides|Primordial}}
| {{element cell property|Np|neptunium | |Actinides|Synthetic}}
| {{element cell property|Pu|plutonium | |Actinides|Synthetic}}
| {{element cell property|Am|americium | |Actinides|Synthetic}}
| {{element cell property|Cm|curium | |Actinides|Synthetic}}
| {{element cell property|Bk|berkelium | |Actinides|Synthetic}}
| {{element cell property|Cf|californium | |Actinides|Synthetic}}
| {{element cell property|Es|einsteinium | |Actinides|Synthetic}}
| {{element cell property|Fm|fermium | |Actinides|Synthetic}}
| {{element cell property|Md|mendelevium | |Actinides|Synthetic}}
| {{element cell property|No|nobelium | |Actinides|Synthetic}}
| {{element cell property|Lr|lawrencium | |Actinides|Synthetic}}
|}
{| border="2" cellpadding="4" style="background:silver; border:1px solid gray; border-collapse:collapse; margin:0 1em 1em 0; text-align:center; {{{1|}}}"
|-
|+ '''[[Chemical series]] of the [[periodic table]]'''
|-
| style="background:{{Element color/Alkali metals}};" width="20%" | [[Alkali metal]]s <!-- also Element color/1 -->
| style="background:{{Element color/Alkaline earth metals}};" width="20%" | [[Alkaline earth metal]]s <!-- also Element color/2 -->
| style="background:{{Element color/Lanthanides}};" width="20%" | [[Lanthanide]]s <!-- also Element color/3 -->
| style="background:{{Element color/Actinides}};" width="20%" | [[Actinide]]s
| style="background:{{Element color/Transition metals}};" width="20%" | [[Transition metal]]s
|-
| style="background:{{Element color/Poor metals}};" | [[Poor metal]]s <!-- also Element color/4 -->
| style="background:{{Element color/Metalloids}};" | [[Metalloid]]s <!-- also Element color/5 -->
| style="background:{{Element color/Nonmetals}};" | [[Nonmetal]]s <!-- also Element color/6 -->
| style="background:{{Element color/Halogens}};" | [[Halogen]]s <!-- also Element color/7 -->
| style="background:{{Element color/Noble gases}};" | [[Noble gas]]es <!-- also Element color/0 -->
|}
</center>
<!-- for a plain white background, use Element color/X -->
==[[Periodic trends]]==
''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 group 7A atom releases more energy than a group 1A atom on gaining an electron because it obtains a filled valence shell.
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, and thus would experience a lesser effective nuclear charge. However, a clear counterexample to this trend can be found in group 2A, and this trend only applies to group 1A atoms.
==Molecular electron affinities==
''E''<sub>ea</sub> is not limited to the elements but also applies to molecules. 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>.
==See also==
*[[Koopmans' theorem]]
*[[One-electron reduction]]
*[[Ionization potential]]
*[[Electronegativity]]
*[[Valence electron]]
==References==
<references/>
*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==
*[http://www.iupac.org/goldbook/E01977.pdf Electron affinity], definition from the [[IUPAC]] [[Gold Book]]
[[Category:Atomic physics]]
[[Category:Chemical bonding]]
[[Category:Chemical properties]]
[[Category:Ions]]
[[Category:Molecular physics]]
[[Category:Physical chemistry]]
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