Naturally occurring
lithiumLithium is a soft, silver-white metal that belongs to the alkali metal group of chemical elements. It is represented by the symbol Li, and it has the atomic number three. Under standard conditions it is the lightest metal and the least dense solid element. Like all alkali metals, lithium is highly...
(
Li) (standard atomic mass: 6.941(2) u) is composed of two stable
isotopeIsotopes are different types of atoms of the same chemical element, each having a different number of neutrons. Correspondingly, isotopes differ in mass number but not in atomic number. The difference in the number of nucleons comes from a difference how many neutrons are in the atomic nucleus...
s ( and , the latter being the more abundant (92.5%
natural abundanceIn chemistry, natural abundance refers to the abundance isotopes of a chemical element as naturally found on a planet. The relative atomic mass of these isotopes is the atomic weight listed for the element in the periodic table...
). Both natural isotopes have anomalously low nuclear binding energy per nucleon compared to the next lighter and heavier elements
HeliumHelium is the chemical element with atomic number 2, and is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert monatomic gas that heads the noble gas group in the periodic table...
and
BerylliumBeryllium is the chemical element with the symbol Be and atomic number 4.A bivalent element, beryllium is found naturally only combined with other elements in minerals. Notable gemstones which contain beryllium include Beryl and Chrysoberyl...
, which means that alone among stable light elements, Lithium can produce net energy through
nuclear fissionIn nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts, often producing free neutrons and lighter nuclei, which may eventually produce photons...
. Seven radioisotopes have been characterized, the most stable being with a
half-lifeHalf-life is the period of time, for a substance undergoing decay, to decrease by half. The name originally was used to describe a characteristic of unstable atoms , but may apply to any quantity which follows a set-rate decay....
of and with a half-life of .
Naturally occurring
lithiumLithium is a soft, silver-white metal that belongs to the alkali metal group of chemical elements. It is represented by the symbol Li, and it has the atomic number three. Under standard conditions it is the lightest metal and the least dense solid element. Like all alkali metals, lithium is highly...
(
Li) (standard atomic mass: 6.941(2) u) is composed of two stable
isotopeIsotopes are different types of atoms of the same chemical element, each having a different number of neutrons. Correspondingly, isotopes differ in mass number but not in atomic number. The difference in the number of nucleons comes from a difference how many neutrons are in the atomic nucleus...
s ( and , the latter being the more abundant (92.5%
natural abundanceIn chemistry, natural abundance refers to the abundance isotopes of a chemical element as naturally found on a planet. The relative atomic mass of these isotopes is the atomic weight listed for the element in the periodic table...
). Both natural isotopes have anomalously low nuclear binding energy per nucleon compared to the next lighter and heavier elements
HeliumHelium is the chemical element with atomic number 2, and is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert monatomic gas that heads the noble gas group in the periodic table...
and
BerylliumBeryllium is the chemical element with the symbol Be and atomic number 4.A bivalent element, beryllium is found naturally only combined with other elements in minerals. Notable gemstones which contain beryllium include Beryl and Chrysoberyl...
, which means that alone among stable light elements, Lithium can produce net energy through
nuclear fissionIn nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts, often producing free neutrons and lighter nuclei, which may eventually produce photons...
. Seven radioisotopes have been characterized, the most stable being with a
half-lifeHalf-life is the period of time, for a substance undergoing decay, to decrease by half. The name originally was used to describe a characteristic of unstable atoms , but may apply to any quantity which follows a set-rate decay....
of and with a half-life of . All of the remaining radioactive isotopes have half-lives that are shorter than . The shortest-lived isotope of lithium is which decays through
proton emissionProton emission is a type of radioactive decay in which a proton is ejected from a nucleus. Proton emission can occur from high-lying excited states in a nucleus following a beta decay, in which case the process is known as beta-delayed proton emission, or can occur from the ground state of very...
and has a half-life of .
is one of the
primordial elementsIn geochemistry, Primordial elements are chemical elements found on the earth that have existed in their current form since before the earth was formed, according to accepted stellar evolution theory...
or, more properly, primordial isotopes, produced in
Big Bang nucleosynthesisIn physical cosmology, Big Bang nucleosynthesis refers to the production of nuclei other than those of H-1 during the early phases of the universe...
(a small amount of is also produced in stars). Lithium isotopes fractionate substantially during a wide variety of natural processes, including mineral formation (chemical precipitation),
metabolismMetabolism is the set of chemical reactions that occur in living organisms to maintain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories. Catabolism breaks down organic matter,...
, and
ion exchangeIon exchange is an exchange of ions between two electrolytes or between an electrolyte solution and a complex. In most cases the term is used to denote the processes of purification, separation, and decontamination of aqueous and other ion-containing solutions with solid polymeric or mineralic 'ion...
. Lithium ion substitutes for
magnesiumMagnesium is a chemical element with the symbol Mg, atomic number 12 and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the earth's crust by mass, although ninth in the Universe as a whole...
and
ironIron is a metallic chemical element with the symbol Fe and atomic number 26. Iron is a group 8 and period 4 element and is therefore classified as a transition metal. Iron and iron alloys are by far the most common metals and the most common ferromagnetic materials in everyday use...
in octahedral sites in
clayClay is a naturally occurring material composed primarily of fine-grained minerals, which show plasticity through a variable range of water content, and which can be hardened when dried and/or fired...
minerals, where is preferred to , resulting in enrichment of the light isotope in processes of hyperfiltration and rock alteration.
Colex separation
Lithium-6 has a greater affinity for mercury than does lithium-7. When a lithium-mercury amalgam is in contact with a lithium hydroxide solution, lithium-6 preferentially concentrates in the amalgam, and lithium-7 in the hydroxide.
This is the basis of the colex (column exchange) separation method, in which a counter-flow of amalgam and hydroxide passes through a cascade of stages. The lithium-6 fraction is preferentially drained by the mercury whereas the lithium-7 fraction flows preferentially with the hydroxide.
At the bottom of the column, the lithium (enriched in lithium-6) is separated from the amalgam, the mercury is recovered and reused with fresh feedstock. At the top, the lithium hydroxide solution is electrolyzed to liberate the lithium-7-enriched fraction. The enrichment obtained with this method varies with the column length and the flow speed.
This method leads to mercury pollution lost in wastes, spills, and through evaporation.
Vacuum distillation
Lithium is heated to a temperature of about 550 °C in a vacuum. Lithium atoms evaporate from the liquid surface and are collected on a cold surface positioned a few centimetres above the liquid surface. Since lithium-6 atoms have a greater mean free path, they are collected preferentially.
The theoretical separation efficiency is about 8%. A multi-stage process may be used to obtain higher degrees of separation.
Lithium-4
Lithium-4 contains 3
protonThe proton is a subatomic particle with an electric charge of +1 elementary charge. It is found in the nucleus of each atom but is also stable by itself and has a second identity as the hydrogen ion, H
+...
s and one
neutronThe neutron is a subatomic particle with no net electric charge and a mass slightly larger than that of a proton.Neutron are usually found in atomic nuclei. The nuclei of most atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of protons in a...
. It is the shortest-lived isotope of lithium. It decays by
proton emissionProton emission is a type of radioactive decay in which a proton is ejected from a nucleus. Proton emission can occur from high-lying excited states in a nucleus following a beta decay, in which case the process is known as beta-delayed proton emission, or can occur from the ground state of very...
and has a half-life of . It can be formed as an intermediate in some nuclear fusion reactions.
Lithium-6
Lithium-6 is valued as a source material for
tritiumTritium is a radioactive isotope of hydrogen. The nucleus of tritium contains one proton and two neutrons, whereas the nucleus of protium contains one proton and no neutrons...
production and as a neutron absorber in
nuclear fusionIn nuclear physics and nuclear chemistry, nuclear fusion is the process by which multiple like-charged atomic nuclei join together to form a heavier nucleus...
. Natural lithium contains about 7.5 percent lithium-6. Large amounts of lithium-6 have been isotopically fractionated for use in
nuclear weaponA nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission or a combination of fission and fusion...
s.
Lithium-7
Some of the material remaining from the production of lithium-6, which is depleted in lithium-6 and enriched in
lithium-7, is made commercially available, and some has been released into the environment. Relative lithium-7 abundances as high as 35.4% greater than the natural value have been measured in ground water from a carbonate aquifer underlying West Valley Creek,
PennsylvaniaThe Commonwealth of Pennsylvania , often colloquially referred to as PA by natives and Northeasterners, is a state located in the Northeastern and Middle Atlantic regions of the United States...
(USA), down-gradient from a lithium processing plant. In depleted material, the relative abundance may be reduced by as much as 80% of its normal value, giving the
atomic massThe atomic mass is the mass of an atom, most often expressed in unified atomic mass units. The atomic mass may be considered to be the total mass of protons, neutrons and electrons in a single atom...
a range from to more than . As a result, the isotopic composition of lithium is highly variable depending on its source. An accurate relative atomic mass cannot be given representatively for all samples.
Lithium-7 finds a use as a constituent of the solvent
lithium fluorideLithium fluoride is a chemical compound of lithium and fluorine. It is a white, inorganic, crystalline, ionic, solid salt under standard conditions. It transmits ultraviolet radiation more efficiently than any other known substance. Uses include specialized UV optics, and as a means to record gamma...
in liquid-fluoride nuclear reactors. Indeed, the large neutron absorption cross-section of lithium-6 (941
barnA barn is a unit of area. Originally used in nuclear physics for expressing the cross sectional area of nuclei and nuclear reactions, today it is used in all fields of high energy physics to express the cross sections of any scattering process...
s, thermal) versus the small neutron absorption cross-section of lithium-7 (0.045 barns, thermal) make strict isotopic separation of lithium a requirement for fluoride reactor use.
Lithium-7 hydroxideLithium hydroxide is a corrosive alkali hydroxide. It is a white hygroscopic crystalline material. It is soluble in water, and slightly soluble in ethanol. It is available commercially in anhydrous form, or as the monohydrate.-Applications:...
is used for alkalizing of the coolant in
pressurized water reactorPressurized water reactors comprise a majority of all western nuclear power plants and are one of two types of light water reactor , the other type being boiling water reactors . In a PWR the primary coolant is pumped under high pressure to the reactor core, then the heated water transfers...
s.
Table
nuclide
symbol |
|
|
isotopic mass (u)
|
half-life |
nuclear
spin |
representative
isotopic
composition
(mole fraction) |
range of natural
variation
(mole fraction) |
| excitation energy |
|
| 3 |
1 |
|
| 2- |
|
|
|
| 3 |
2 |
|
| 3/2- |
|
|
|
| 3 |
3 |
| STABLE |
1+ |
|
|
|
| 3 |
4 |
| STABLE |
3/2- |
|
|
|
| 3 |
5 |
|
| 2+ |
|
|
|
| 3 |
6 |
|
| 3/2- |
|
|
|
| 3 |
7 |
|
| (1-,2-) |
|
|
| |
|
| 1+ |
|
|
| |
|
| 2+ |
|
|
|
| 3 |
8 |
|
| 3/2- |
|
|
|
| 3 |
9 |
|
| |
|
|