Electron affinities

Electron affinityFrom Wikipedia, the free encyclopediaJump to:
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The Electron affinity of a molecule
<http://en.wikipedia.org/wiki/Molecule>  or atom
<http://en.wikipedia.org/wiki/Atom>    is the energy change when an
electron is added to the neutral atom to   form a negative ion. This
property can only be measured in an atom in   gaseous state.
X + eâˆ' â†' Xâˆ'The electron affinity, Eea, is defined
as positive when the resulting ion has a lower energy, i.e. it is an
exothermic process <http://en.wikipedia.org/wiki/Exothermic_reaction> 
that releases energy:
Eea = Einitial âˆ' EfinalAlternately, electron affinity is often
described as the amount of energy required to detach an electron from a
singly charged <http://en.wikipedia.org/wiki/Electric_charge>  negative
ion <http://en.wikipedia.org/wiki/Ion> , i.e. the energy change for the
process
Xâˆ' â†' X + eâˆ'A molecule or atom that has a positive
electron affinity is often called an electron acceptor
<http://en.wikipedia.org/wiki/Electron_acceptor>  and may undergo
charge-transfer <http://en.wikipedia.org/wiki/Charge-transfer> 
reactions.
Electron affinities of the elementsMain article: Electron affinity (data
page) <http://en.wikipedia.org/wiki/Electron_affinity_%28data_page%29>
Although Eea varies greatly across the periodic table, some patterns
emerge. Generally, nonmetals <http://en.wikipedia.org/wiki/Nonmetals> 
have more positive Eea than metals <http://en.wikipedia.org/wiki/Metal>
. Atoms whose anions are more stable than neutral atoms have a greater
Eea. Chlorine <http://en.wikipedia.org/wiki/Chlorine>  most strongly
attracts extra electrons; mercury
<http://en.wikipedia.org/wiki/Mercury_%28element%29>    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, 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. 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
<http://en.wikipedia.org/wiki/Joule_per_mole> .   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.

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