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= Francium =
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Introduction
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Francium is a chemical element; it has symbol Fr and atomic number 87.
It is extremely radioactive; its most stable isotope, francium-223
(originally called 'actinium K' after the natural decay chain in which
it appears), has a half-life of only 22 minutes. It is the second-most
electropositive element, behind only caesium, and is the second rarest
naturally occurring element (after astatine). Francium's isotopes
decay quickly into astatine, radium, and radon. The electronic
structure of a francium atom is [Rn] 7s1; thus, the element is classed
as an alkali metal.
As a consequence of its extreme instability, bulk francium has never
been seen. Because of the general appearance of the other elements in
its periodic table column, it is presumed that francium would appear
as a highly reactive metal if enough could be collected together to be
viewed as a bulk solid or liquid. Obtaining such a sample is highly
improbable since the extreme heat of decay resulting from its short
half-life would immediately vaporize any viewable quantity of the
element.
Francium was discovered by Marguerite Perey in France (from which the
element takes its name) on January 7, 1939. Before its discovery,
francium was referred to as 'eka-caesium' or 'ekacaesium' because of
its conjectured existence below caesium in the periodic table. It was
the last element first discovered in nature, rather than by synthesis.
Outside the laboratory, francium is extremely rare, with trace amounts
found in uranium ores, where the isotope francium-223 (in the family
of uranium-235) continually forms and decays. As little as 1 oz exists
at any given time throughout the Earth's crust; aside from
francium-223 and francium-221, its other isotopes are entirely
synthetic. The largest amount produced in the laboratory was a cluster
of more than 300,000 atoms.
Characteristics
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Francium is one of the most unstable of the naturally occurring
elements: its longest-lived isotope, francium-223, has a half-life of
only 22 minutes. The only comparable element is astatine, whose most
stable natural isotope, astatine-219 (the alpha daughter of
francium-223), has a half-life of 56 seconds, although synthetic
astatine-210 is much longer-lived with a half-life of 8.1 hours. All
isotopes of francium decay into astatine, radium, or radon.
Francium-223 also has a shorter half-life than the longest-lived
isotope known of each element up to and including element 105,
dubnium.
Francium is an alkali metal whose chemical properties mostly resemble
those of caesium. A heavy element with a single valence electron, it
has the highest equivalent weight of any element. Liquid francium--if
created--should have a surface tension of 0.05092 N/m at its melting
point. Francium's melting point was estimated to be around 8.0 C; a
value of 27 C is also often encountered. The melting point is
uncertain because of the element's extreme rarity and radioactivity; a
different extrapolation based on Dmitri Mendeleev's method gave 20 ±.
A calculation based on the melting temperatures of binary ionic
crystals gives 24.861 ±. The estimated boiling point of 620 C is also
uncertain; the estimates 598 C and 677 C, as well as the extrapolation
from Mendeleev's method of 640 C, have also been suggested. The
density of francium is expected to be around 2.48 g/cm3 (Mendeleev's
method extrapolates 2.4 g/cm3).
Linus Pauling estimated the electronegativity of francium at 0.7 on
the Pauling scale, the same as caesium; the value for caesium has
since been refined to 0.79, but there are no experimental data to
allow a refinement of the value for francium. Francium has a slightly
higher ionization energy than caesium, 392.811(4) kJ/mol as opposed to
375.7041(2) kJ/mol for caesium, as would be expected from relativistic
effects, and this would imply that caesium is the less electronegative
of the two. Francium should also have a higher electron affinity than
caesium and the Fr− ion should be more polarizable than the Cs− ion.
Compounds
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As a result of francium's instability, its salts are only known to a
small extent. Francium coprecipitates with several caesium salts, such
as caesium perchlorate, which results in small amounts of francium
perchlorate. This coprecipitation can be used to isolate francium, by
adapting the radiocaesium coprecipitation method of Lawrence E.
Glendenin and C. M. Nelson. It will additionally coprecipitate with
many other caesium salts, including the iodate, the picrate, the
tartrate (also rubidium tartrate), the chloroplatinate, and the
silicotungstate. It also coprecipitates with silicotungstic acid, and
with perchloric acid, without another alkali metal as a carrier, which
leads to other methods of separation.
Francium perchlorate
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Francium perchlorate is produced by the reaction of francium chloride
and sodium perchlorate. The francium perchlorate coprecipitates with
caesium perchlorate. This coprecipitation can be used to isolate
francium, by adapting the radiocaesium coprecipitation method of
Lawrence E. Glendenin and C. M. Nelson. However, this method is
unreliable in separating thallium, which also coprecipitates with
caesium. Francium perchlorate's entropy is expected to be 42.7 e.u
(178.7 J mol−1 K−1).
Francium halides
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Francium halides are all soluble in water and are expected to be white
solids. They are expected to be produced by the reaction of the
corresponding halogens. For example, francium chloride would be
produced by the reaction of francium and chlorine. Francium chloride
has been studied as a pathway to separate francium from other
elements, by using the high vapour pressure of the compound, although
francium fluoride would have a higher vapour pressure.
Other compounds
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Francium nitrate, sulfate, hydroxide, carbonate, acetate, and oxalate,
are all soluble in water, while the iodate, picrate, tartrate,
chloroplatinate, and silicotungstate are insoluble. The insolubility
of these compounds are used to extract francium from other radioactive
products, such as zirconium, niobium, molybdenum, tin, antimony, the
method mentioned in the section above. Francium oxide is believed to
disproportionate to the peroxide and francium metal. The CsFr
molecule is predicted to have the heavier element (francium) at the
negative end of the dipole, unlike all known heterodiatomic alkali
metal molecules. Francium superoxide (FrO2) is expected to have a more
covalent character than its lighter congeners; this is attributed to
the 6p electrons in francium being more involved in the
francium-oxygen bonding. The relativistic destabilisation of the 6p3/2
spinor may make francium compounds in oxidation states higher than +1
possible, such as [FrVF6]−; but this has not been experimentally
confirmed.
Isotopes
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There are 37 known isotopes of francium ranging in atomic mass from
197 to 233. Francium has seven metastable nuclear isomers.
Francium-223 and francium-221 are the only isotopes that occur in
nature, with the former being far more common.
Francium-223 is the most stable isotope, with a half-life of 21.8
minutes, and it is highly unlikely that an isotope of francium with a
longer half-life will ever be discovered or synthesized. Francium-223
is a fifth product of the uranium-235 decay series as a daughter
isotope of actinium-227; thorium-227 is the more common daughter.
Francium-223 then decays into radium-223 by beta decay (1.149 MeV
decay energy), with a minor (0.006%) alpha decay path to astatine-219
(5.4 MeV decay energy).
Francium-221 has a half-life of 4.8 minutes. It is the ninth product
of the neptunium decay series as a daughter isotope of actinium-225.
Francium-221 then decays into astatine-217 by alpha decay (6.457 MeV
decay energy). Although all primordial 237Np is extinct, the neptunium
decay series continues to exist naturally in tiny traces due to (n,2n)
knockout reactions in natural 238U. Francium-222, with a half-life of
14 minutes, may be produced as a result of the beta decay of natural
radon-222; this process has nonetheless not yet been observed, and it
is unknown whether this process is energetically possible.
The least stable ground state isotope is francium-215, with a
half-life of 90 ns: it undergoes a 9.54 MeV alpha decay to
astatine-211.
Applications
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Due to its instability and rarity, there are no commercial
applications for francium. It has been used for research purposes in
the fields of chemistry
and of atomic structure. Its use as a potential diagnostic aid for
various cancers has also been explored, but this application has been
deemed impractical.
Francium's ability to be synthesized, trapped, and cooled, along with
its relatively simple atomic structure, has made it the subject of
specialized spectroscopy experiments. These experiments have led to
more specific information regarding energy levels and the coupling
constants between subatomic particles. Studies on the light emitted by
laser-trapped francium-210 ions have provided accurate data on
transitions between atomic energy levels which are fairly similar to
those predicted by quantum theory. Francium is a prospective candidate
for searching for CP violation.
History
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As early as 1870, chemists thought that there should be an alkali
metal beyond caesium, with an atomic number of 87. It was then
referred to by the provisional name 'eka-caesium'.
Erroneous and incomplete discoveries
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In 1914, Stefan Meyer, Viktor F. Hess, and Friedrich Paneth (working
in Vienna) made measurements of alpha radiation from various
substances, including 227Ac. They observed the possibility of a minor
alpha branch of this nuclide, though follow-up work could not be done
due to the outbreak of World War I. Their observations were not
precise and sure enough for them to announce the discovery of element
87, though it is likely that they did indeed observe the decay of
227Ac to 223Fr.
Soviet chemist Dmitry Dobroserdov was the first scientist to claim to
have found eka-caesium, or francium. In 1925, he observed weak
radioactivity in a sample of potassium, another alkali metal, and
incorrectly concluded that eka-caesium was contaminating the sample
(the radioactivity from the sample was from the naturally occurring
potassium radioisotope, potassium-40). He then published a thesis on
his predictions of the properties of eka-caesium, in which he named
the element 'russium' after his home country. Shortly thereafter,
Dobroserdov began to focus on his teaching career at the Polytechnic
Institute of Odesa, and he did not pursue the element further.
The following year, English chemists Gerald J. F. Druce and Frederick
H. Loring analyzed X-ray photographs of manganese(II) sulfate. They
observed spectral lines which they presumed to be of eka-caesium. They
announced their discovery of element 87 and proposed the name
'alkalinium', as it would be the heaviest alkali metal.
In 1930, Fred Allison of the Alabama Polytechnic Institute claimed to
have discovered element 87 (in addition to 85) when analyzing
pollucite and lepidolite using his magneto-optical machine. Allison
requested that it be named 'virginium' after his home state of
Virginia, along with the symbols Vi and Vm. In 1934, H.G. MacPherson
of UC Berkeley disproved the effectiveness of Allison's device and the
validity of his discovery.
In 1936, Romanian physicist Horia Hulubei and his French colleague
Yvette Cauchois also analyzed pollucite, this time using their
high-resolution X-ray apparatus. They observed several weak emission
lines, which they presumed to be those of element 87. Hulubei and
Cauchois reported their discovery and proposed the name 'moldavium',
along with the symbol Ml, after Moldavia, the Romanian province where
Hulubei was born. In 1937, Hulubei's work was criticized by American
physicist F. H. Hirsh Jr., who rejected Hulubei's research methods.
Hirsh was certain that eka-caesium would not be found in nature, and
that Hulubei had instead observed mercury or bismuth X-ray lines.
Hulubei insisted that his X-ray apparatus and methods were too
accurate to make such a mistake. Because of this, Jean Baptiste
Perrin, Nobel Prize winner and Hulubei's mentor, endorsed moldavium as
the true eka-caesium over Marguerite Perey's recently discovered
francium. Perey took pains to be accurate and detailed in her
criticism of Hulubei's work, and finally she was credited as the sole
discoverer of element 87. All other previous purported discoveries of
element 87 were ruled out due to francium's very limited half-life.
Perey's analysis
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Eka-caesium was discovered on January 7, 1939, by Marguerite Perey of
the Curie Institute in Paris, when she purified a sample of
actinium-227 which had been reported to have a decay energy of 220
keV. Perey noticed decay particles with an energy level below 80 keV.
Perey thought this decay activity might have been caused by a
previously unidentified decay product, one which was separated during
purification, but emerged again out of the pure actinium-227. Various
tests eliminated the possibility of the unknown element being thorium,
radium, lead, bismuth, or thallium. The new product exhibited chemical
properties of an alkali metal (such as coprecipitating with caesium
salts), which led Perey to believe that it was element 87, produced by
the alpha decay of actinium-227. Perey then attempted to determine the
proportion of beta decay to alpha decay in actinium-227. Her first
test put the alpha branching at 0.6%, a figure which she later revised
to 1%.
Perey named the new isotope 'actinium-K' (it is now referred to as
francium-223) and in 1946, she proposed the name 'catium' (Cm) for her
newly discovered element, as she believed it to be the most
electropositive cation of the elements. Irène Joliot-Curie, one of
Perey's supervisors, opposed the name due to its connotation of 'cat'
rather than 'cation'; furthermore, the symbol coincided with that
which had since been assigned to curium. Perey then suggested
'francium', after France. This name was officially adopted by the
International Union of Pure and Applied Chemistry (IUPAC) in 1949,
becoming the second element after gallium to be named after France. It
was assigned the symbol Fa, but it was revised to the current Fr
shortly thereafter. Francium was the last element discovered in
nature, rather than synthesized, following hafnium and rhenium.
Further research into francium's structure was carried out by, among
others, Sylvain Lieberman and his team at CERN in the 1970s and 1980s.
Occurrence
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223Fr is the result of the alpha decay of 227Ac and can be found in
trace amounts in uranium minerals. In a given sample of uranium, there
is estimated to be only one francium atom for every 1 × 1018 uranium
atoms. Only about 1 oz of francium is present naturally in the earth's
crust.
Production
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Francium can be synthesized by a fusion reaction when a gold-197
target is bombarded with a beam of oxygen-18 atoms from a linear
accelerator in a process originally developed at the physics
department of the State University of New York at Stony Brook in 1995.
Depending on the energy of the oxygen beam, the reaction can yield
francium isotopes with masses of 209, 210, and 211.
:197Au + 18O → 209Fr + 6 n
:197Au + 18O → 210Fr + 5 n
:197Au + 18O → 211Fr + 4 n
The francium atoms leave the gold target as ions, which are
neutralized by collision with yttrium and then isolated in a
magneto-optical trap (MOT) in a gaseous unconsolidated state. Although
the atoms only remain in the trap for about 30 seconds before escaping
or undergoing nuclear decay, the process supplies a continual stream
of fresh atoms. The result is a steady state containing a fairly
constant number of atoms for a much longer time. The original
apparatus could trap up to a few thousand atoms, while a later
improved design could trap over 300,000 at a time. Sensitive
measurements of the light emitted and absorbed by the trapped atoms
provided the first experimental results on various transitions between
atomic energy levels in francium. Initial measurements show very good
agreement between experimental values and calculations based on
quantum theory. The research project using this production method
relocated to TRIUMF in 2012, where over 106 francium atoms have been
held at a time, including large amounts of 209Fr in addition to 207Fr
and 221Fr.
Other synthesis methods include bombarding radium with neutrons, and
bombarding thorium with protons, deuterons, or helium ions.
223Fr can also be isolated from samples of its parent 227Ac, the
francium being milked via elution with NH4Cl-CrO3 from an
actinium-containing cation exchanger and purified by passing the
solution through a silicon dioxide compound loaded with barium
sulfate.
In 1996, the Stony Brook group trapped 3000 atoms in their MOT, which
was enough for a video camera to capture the light given off by the
atoms as they fluoresce. Francium has not been synthesized in amounts
large enough to weigh.
External links
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* [
http://www.periodicvideos.com/videos/087.htm Francium] at 'The
Periodic Table of Videos' (University of Nottingham)
* [
http://www.webelements.com/webelements/elements/text/Fr/index.html
WebElements.com - Francium]
*
[
https://web.archive.org/web/19981203125538/http://fr.physics.sunysb.edu/francium_news/frconten.htm
Stony Brook University Physics Dept.]
* Scerri, Eric (2013). 'A Tale of Seven Elements', Oxford University
Press, Oxford,
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Original Article:
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