A solvated electron is a
free electron
The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...
in a
solution, in which it behaves like an
anion
An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...
. An electron's being solvated in a solution means it is bound by the solution. The notation for a solvated electron in formulas of chemical reactions is "e
−". Often, discussions of solvated electrons focus on their solutions in
ammonia
Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...
, which are stable for days, but solvated electrons also occur in water and many other solventsin fact, in any solvent that mediates
outer-sphere electron transfer. Solvated electrons are responsible for a great deal of
radiation chemistry.
Salt
In common usage, salt is a mineral composed primarily of sodium chloride (NaCl). When used in food, especially in granulated form, it is more formally called table salt. In the form of a natural crystalline mineral, salt is also known as r ...
s containing solvated electrons are known as
electrides.
Ammonia solutions
Liquid
ammonia
Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...
will dissolve all of the
alkali metal
The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K),The symbols Na and K for sodium and potassium are derived from their Latin names, ''natrium'' and ''kalium''; these are still the origins of the names ...
s and other
electropositive metals such as
Ca,
Sr,
Ba,
Eu, and
Yb (also
Mg using an electrolytic process), giving characteristic blue solutions. For alkali metals in
liquid ammonia, the solution is blue when dilute and copper-colored when more concentrated (> 3
molar).
These solutions
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.
Solvated electrons in ammonia are the anions of salts called
electrides.
:Na + 6 NH
3 →
3)6">a(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 volume resistivity or specific electrical resistance) is a fundamental specific property of a material that measures its electrical resistance or how strongly it resists electric current. A low resistivity in ...
increases from 10
−2 to 10
4 Ω−1 cm−1 (larger than liquid
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
Density (volumetric mass density or specific mass) is the ratio of a substance's mass to its volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' (or ''d'') can also be u ...
decreases by 30%.
Other solvents
Alkali metals also dissolve in some small
primary amines, such as
methylamine and
ethylamine and
hexamethylphosphoramide, forming blue solutions.
Tetrahydrofuran (THF) dissolves alkali metal, but a
Birch reduction (see ) analogue does not proceed without a
diamine ligand
In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's el ...
. 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 though the solvated electron has only a fleeting existence. Below pH = 9.6 the hydrated electron reacts with the
hydronium ion giving atomic hydrogen, which in turn can react with the hydrated electron giving hydroxide ion and usual molecular hydrogen H
2.
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 is a suspension (chemistry), suspension of fine solid particles or liquid Drop (liquid), droplets in air or another gas. Aerosols can be generated from natural or Human impact on the environment, human causes. The term ''aerosol'' co ...
formation.
Its
standard electrode potential
In electrochemistry, standard electrode potential E^\ominus, or E^\ominus_, is the electrode potential (a measure of the reducing power of any element or compound) which the IUPAC "Gold Book" defines as ''"the value of the standard emf ( electrom ...
value is −2.88 V. The equivalent conductivity of 177 Mho cm
2 is similar to that of
hydroxide ion
Hydroxide is a polyatomic ion, diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually Self-ionization ...
. This value of equivalent conductivity corresponds to a diffusivity of 4.75
cm
2s
−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:
:2
3)6">a(NH3)6sup>+e
− → H
2 + 2 NaNH
2 + 10 NH
3
Electride salts can be isolated by the addition of
macrocyclic ligand
In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's el ...
s such as
crown ether and
cryptands to solutions containing solvated electrons. These ligands strongly bind the cations and prevent their re-reduction by the electron.
:
3)6">a(NH3)6sup>+e
− + cryptand →
a(cryptand)sup>+e
−+ 6 NH
3
The solvated electron reacts with
oxygen
Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...
to form a
superoxide
In chemistry, a superoxide is a compound that contains the superoxide ion, which has the chemical formula . The systematic name of the anion is dioxide(1−). The reactive oxygen ion superoxide is particularly important as the product of t ...
radical (O
2.−). With
nitrous oxide
Nitrous oxide (dinitrogen oxide or dinitrogen monoxide), commonly known as laughing gas, nitrous, or factitious air, among others, is a chemical compound, an Nitrogen oxide, oxide of nitrogen with the Chemical formula, formula . At room te ...
, solvated electrons react to form
nitroxyl radicals (NO
.).
Uses
Solvated electrons are involved in electrode processes, a broad area with many technical applications (
electrosynthesis,
electroplating
Electroplating, also known as electrochemical deposition or electrodeposition, is a process for producing a metal coating on a solid substrate through the redox, reduction of cations of that metal by means of a direct current, direct electric cur ...
,
electrowinning).
A specialized use of sodium-ammonia solutions is the
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.
Work by Cullen ''et al.'' showed that metal-ammonia solutions can be used to intercalate a range of layered materials, which can then be exfoliated in polar, aprotic solvents, to produce ionic solutions of two-dimensional materials. An example of this is the intercalation of graphite with potassium and ammonia, which is then exfoliated by spontaneous dissolution in THF to produce a graphenide solution.
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 British chemist and inventor who invented the Davy lamp and a very early form of arc lamp. He is also remembered for isolating, by using electricity, several Chemical element, e ...
. In 1807–1809, he examined the addition of grains of potassium to gaseous ammonia (liquefaction of ammonia was invented in 1823).
James Ballantyne Hannay 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 reciprocal quantity is , measuring the ease with which an electric current passes. Electrical resistance shares some conceptual paral ...
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 that different metals and different solvents (
methylamine,
ethylamine) 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
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Solutions
Nuclear chemistry
Organic chemistry
Radiation
Electrides