hydrated electron
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A solvated electron is a
free Free may refer to: Concept * Freedom, having the ability to act or change without constraint * Emancipate, to procure political rights, as for a disenfranchised group * Free will, control exercised by rational agents over their actions and decis ...

free
electron The electron is a subatomic particle (denoted by the symbol or ) whose electric charge is negative one elementary charge. Electrons belong to the first generation (particle physics), generation of the lepton particle family, and are general ...

electron
in (solvated in) a
solution Solution may refer to: * Solution (chemistry) Image:SaltInWaterSolutionLiquid.jpg, upMaking a saline water solution by dissolving Salt, table salt (sodium chloride, NaCl) in water. The salt is the solute and the water the solvent. In chemistry ...
, and is the smallest possible
anion An ion () is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ...
. Solvated electrons occur widely. Often, discussions of solvated electrons focus on their solutions in ammonia, which are stable for days, but solvated electrons also occur in water and other solvents in fact, in any solvent that mediates outer-sphere electron transfer. The solvated electron is responsible for a great deal of
radiation chemistryRadiation chemistry is a subdivision of nuclear chemistry which is the study of the chemical effects of radiation on matter; this is very different from radiochemistry as no radioactivity needs to be present in the material which is being chemically ...
.


Ammonia solutions

Liquid ammonia will dissolve all of the
alkali metal The alkali metals consist of the chemical element In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structur ...
s and other
electropositive Electronegativity, symbolized as ''Chi (letter), χ'', is the tendency for an atom of a given chemical element to attract shared electrons (or electron density) when forming a chemical bond. An atom's electronegativity is affected by both its ato ...

electropositive
metals such as
Ca
Ca
,
Sr
Sr
,
Ba
Ba
,
Eu
Eu
, and
Yb
Yb
(also
Mg
Mg
using an electrolytic process), giving characteristic blue solutions. For alkali metals in
liquid ammonia Ammonia is a chemical compound, compound of nitrogen and hydrogen with the chemical formula, formula NH3. A Binary compounds of hydrogen, stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct ch ...
, the solution is blue when dilute and copper-colored when more concentrated (> 3 molar). These solutions
conduct electricity
conduct electricity
. The blue colour of the solution is due to ammoniated electrons, which absorb energy in the visible region of light. The diffusivity of the solvated electron in liquid ammonia can be determined using potential-step
chronoamperometry Chronoamperometry is an electrochemical technique in which the potential of the working electrode is stepped and the resulting current from faradaic processes occurring at the electrode (caused by the potential step) is monitored as a function of t ...
. Solvated electrons in ammonia are the anions of salts called
electride An electride is an ionic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior ...
s. :Na + 6 NH3a(NH3)6sup>+e- The reaction is reversible: evaporation of the ammonia solution produces a film of metallic sodium.


Case study: Li in NH3

A lithium–ammonia solution at −60 °C is saturated at about 15 mol% metal (MPM). When the concentration is increased in this range
electrical conductivity Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that measures how strongly it resists electric current An electric current is a stream of charged particle In p ...
increases from 10−2 to 104 
ohm The ohm (symbol: Ω) is the SI derived unit SI derived units are units of measurement derived from the seven SI base unit, base units specified by the International System of Units (SI). They are either dimensionless quantity, dimensionless o ...

ohm
−1 cm−1 (larger than liquid
mercury Mercury usually refers to: * Mercury (planet) Mercury is the smallest planet in the Solar System and the closest to the Sun. Its orbit around the Sun takes 87.97 Earth days, the shortest of all the Sun's planets. It is named after the Roman g ...

mercury
). At around 8 MPM, a "transition to the metallic state" (TMS) takes place (also called a "metal-to-nonmetal transition" (MNMT)). At 4 MPM a liquid-liquid phase separation takes place: the less dense gold-colored phase becomes immiscible from a denser blue phase. Above 8 MPM the solution is bronze/gold-colored. In the same concentration range the overall
density The density (more precisely, the volumetric mass density; also known as specific mass), of a substance is its per unit . The symbol most often used for density is ''ρ'' (the lower case Greek letter ), although the Latin letter ''D'' can also ...

density
decreases by 30%.


Other solvents

Alkali metals also dissolve in some small
primary amine In organic chemistry Organic chemistry is a branch of chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, ...
s, such as
methylamine Methylamine is an organic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavio ...

methylamine
and
ethylamine Ethylamine is an organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bond, bonds. Due to carbon's ability to Catenation, caten ...
and
hexamethylphosphoramide Hexamethylphosphoramide, often abbreviated HMPA, is a phosphoramide (an amide of phosphoric acid) with the chemical formula, formula This colorless liquid is a useful reagent in organic synthesis. Structure and reactivity HMPA is the oxide of th ...
, forming blue solutions. THF solutions of diamines are effective. Solvated electron solutions of the alkaline earth metals magnesium, calcium, strontium and barium in ethylenediamine have been used to intercalate graphite with these metals.


Water

Solvated electrons are involved in the reaction of alkali metals with water, even thought the solvated electron has only a fleeting existence. Below pH = 9.6 the hydrated electron reacts with the
hydronium In chemistry Chemistry is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in th ...

hydronium
ion giving atomic hydrogen, which in turn can react with the hydrated electron giving hydroxide ion and usual molecular hydrogen H2. Solvated electrons can be found even in the gas phase. This implies their possible existence in the upper atmosphere of Earth and involvement in nucleation and
aerosol An aerosol (abbreviation of "aero-solution") is a suspension Suspension or suspended may refer to: Science and engineering * Suspension (topology), in mathematics * Suspension (dynamical systems), in mathematics * Suspension of a ring, in math ...

aerosol
formation. Its
standard electrode potential In electrochemistry Electrochemistry is the branch of physical chemistry Physical chemistry is the study of macroscopic The macroscopic scale is the length scale on which objects or phenomena are large enough to be visible with the naked ey ...
value is -2.77 V. Equivalent conductivity 177 Mho cm2 is similar to that of
hydroxide ion Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen Oxygen is the chemical element with the chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen Group (periodic table), group ...
. This value of equivalent conductivity corresponds to a diffusivity of 4,75*10−5 cm2s−1.


Reactivity

Although quite stable, the blue ammonia solutions containing solvated electrons degrade rapidly in the presence of catalysts to give colorless solutions of
sodium amide Sodium amide, commonly called sodamide (systematic name sodium azanide), is the inorganic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of ...
: :2 a(NH3)6sup>+e- → H2 + 2 NaNH2 + 10 NH3 Electride salts can be isolated by the addition of
macrocyclic Macrocycles are often described as molecules and ions containing a twelve or more membered ring. Classical examples include the crown ether Crown ethers are cyclic chemical compound A chemical compound is a chemical substance A chemical su ...
ligand In coordination chemistry A coordination complex consists of a central atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by havi ...
s such as
crown ether Crown ethers are cyclic chemical compound A chemical compound is a chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by havin ...

crown ether
and
cryptand Cryptands are a family of synthetic bicyclic and polycyclic multidentate ligand In coordination chemistry, a ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex. The bonding with t ...

cryptand
s to solutions containing solvated electrons. These ligands strongly bind the cations and prevent their re-reduction by the electron. : a(NH3)6sup>+e- + cryptand → a(cryptand)sup>+e-+ 6 NH3 The solvated electron reacts with
oxygen Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

oxygen
to form a
superoxide A superoxide is a compound that contains the superoxide ion An ion () is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space ...

superoxide
radical Radical may refer to: Arts and entertainment Music *Radical (mixtape), ''Radical'' (mixtape), by Odd Future, 2010 *Radical (Smack album), ''Radical'' (Smack album), 1988 *"Radicals", a song by Tyler, The Creator from the 2011 album ''Goblin (album ...
(O2.−). With
nitrous oxide Nitrous oxide, commonly known as laughing gas, nitrous, or nos, is a chemical compound A chemical compound is a chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is an ...
, solvated electrons react to form
hydroxyl A hydroxy or hydroxyl group is a functional group with the chemical formula -OH and composed of one oxygen Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the ...

hydroxyl
radicals (HO.).


Applications

Solvated electrons are involved in electrode processes, a broad area with many technical applications (electrosynthesis, electroplating, electrowinning). A speciallized use of sodium-ammonia solutions is the
Birch reduction The Birch reduction is an organic reaction that is used to convert arenes to cyclohexadienes. The reaction is named after the Australian chemist Arthur Birch (organic chemist), Arthur Birch and involves the organic reduction of aromatic rings in li ...

Birch reduction
. Other reactions where sodium is used as a reducing agent also are assumed to involve solvated electrons, e.g. the use of sodium in ethanol as in the
Bouveault–Blanc reduction The Bouveault–Blanc reduction is a chemical reaction in which an ester is redox, reduced to primary alcohols using absolute ethanol and sodium metal. It was first reported by Louis Bouveault and Gustave Louis Blanc in 1903. Bouveault and Blanc d ...
.


History

The observation of the color of metal-electride solutions is generally attributed to
Humphry Davy Sir Humphry Davy, 1st Baronet (17 December 177829 May 1829) was a Cornish Cornish is the adjective and demonym associated with Cornwall, the most southwesterly part of the United Kingdom. It may refer to: * Cornish language, a Brittonic South ...

Humphry Davy
. In 1807–1809, he examined the addition of grains of potassium to gaseous ammonia (liquefaction of ammonia was invented in 1823).
James Ballantyne Hannay James Ballantyne Hannay FRSE Fellowship of the Royal Society of Edinburgh (FRSE) is an award granted to individuals that the Royal Society of Edinburgh, Scotland's national academy of science and Literature, letters, judged to be "eminently dis ...
and J. Hogarth repeated the experiments with sodium in 1879–1880. W. Weyl in 1864 and C. A. Seely in 1871 used liquid ammonia, whereas Hamilton Cady in 1897 related the ionizing properties of ammonia to that of water. Charles A. Kraus measured the
electrical conductance The electrical resistance of an object is a measure of its opposition to the flow of electric current. Its Multiplicative inverse, reciprocal quantity is , measuring the ease with which an electric current passes. Electrical resistance shares so ...
of metal ammonia solutions and in 1907 attributed it to the electrons liberated from the metal. In 1918, G. E. Gibson and W. L. Argo introduced the solvated electron concept. They noted based on
absorption spectra Absorption may refer to: Chemistry and biology *Absorption (chemistry), diffusion of particles of gas or liquid into liquid or solid materials *Absorption (skin), a route by which substances enter the body through the skin *Absorption (pharmacolo ...
that different metals and different solvents (
methylamine Methylamine is an organic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavio ...

methylamine
,
ethylamine Ethylamine is an organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bond, bonds. Due to carbon's ability to Catenation, caten ...
) produce the same blue color, attributed to a common species, the solvated electron. In the 1970s, solid salts containing electrons as the anion were characterized.


References


Further reading

* * * * * * * *
The electrochemistry of the solvated electron
Technische Universiteit Eindhoven.
IAEA On the Electrolytic Generation of Hydrated Electron
* Fundamentals of Radiation Chemistry, chapter 6
p. 145–198
Academic Press, 1999.
Tables of bimolecular rate constants of hydrated electrons, hydrogen atoms and hydroxyl radicals with inorganic and organic compounds International Journal of Applied Radiation and Isotopes
Anbar, Neta] {{refend Solutions Nuclear chemistry Organic chemistry Radiation Electrides