Francium is a

chemical element A chemical element is a species of atoms that have a given number of protons in their atomic nucleus, nuclei, including the pure Chemical substance, substance consisting only of that species. Unlike chemical compounds, chemical elements canno ...

with the

symbol A symbol is a mark, sign, or word that indicates, signifies, or is understood as representing an idea, object, or relationship. Symbols allow people to go beyond what is known or seen by creating linkages between otherwise very different conc ...

Fr and

atomic number The atomic number or nuclear charge number (symbol ''Z'') of a chemical element is the charge number of an atomic nucleus. For ordinary nuclei, this is equal to the proton number (''n''p) or the number of protons found in the nucleus of ever ...

87. It is extremely

radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consi ...

; its most stable isotope, francium-223 (originally called

actinium Actinium is a chemical element with the symbol Ac and atomic number 89. It was first isolated by Friedrich Oskar Giesel in 1902, who gave it the name ''emanium''; the element got its name by being wrongly identified with a substance ...

K after the natural decay chain it appears in), has a half-life of only 22 minutes. It is the second-most electropositive element, behind only

caesium Caesium (IUPAC spelling) (or cesium in American English) is a chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-golden alkali metal with a melting point of , which makes it one of only five elemental metals that a ...

, and is the second rarest naturally occurring element (after

astatine Astatine is a chemical element with the symbol At and atomic number 85. It is the rarest naturally occurring element in the Earth's crust, occurring only as the decay product of various heavier elements. All of astatine's isotopes are short-live ...

). The isotopes of francium decay quickly into astatine,

radium Radium is a chemical element with the symbol Ra and atomic number 88. It is the sixth element in group 2 of the periodic table, also known as the alkaline earth metals. Pure radium is silvery-white, but it readily reacts with nitrogen (rat ...

, and

radon Radon is a chemical element with the symbol Rn and atomic number 86. It is a radioactive, colourless, odourless, tasteless noble gas. It occurs naturally in minute quantities as an intermediate step in the normal radioactive decay chains th ...

. The electronic structure of a francium atom is n7s¹, and so the element is classed as an

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 ...

. 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) in 1939. Prior to its discovery, it was referred to as '' eka-

'' 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 Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weak ...

ores, where the

isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers ( mass num ...

francium-223 (in the family of uranium-235) continually forms and decays. As little as 200–500 g exists at any given time throughout the

Earth's crust Earth's crust is Earth's thin outer shell of rock, referring to less than 1% of Earth's radius and volume. It is the top component of the lithosphere, a division of Earth's layers that includes the crust and the upper part of the mantle. The ...

; 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

Francium is one of the most unstable of the naturally occurring elements: its longest-lived isotope, francium-223, has a

half-life Half-life (symbol ) is the time required for a quantity (of substance) to reduce to half of its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable ...

of only 22 minutes. The only comparable element is

, 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,

, or

. Francium-223 also has a shorter half-life than the longest-lived isotope of each synthetic 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 In chemistry, equivalent weight (also known as gram equivalent) is the mass of one equivalent, that is the mass of a given substance which will combine with or displace a fixed quantity of another substance. The equivalent weight of an element i ...

of any element. Liquid francium—if created—should have a

surface tension Surface tension is the tendency of liquid surfaces at rest to shrink into the minimum surface area possible. Surface tension is what allows objects with a higher density than water such as razor blades and insects (e.g. water striders) t ...

of 0.05092 N/m at its melting point. Francium's melting point was estimated to be around ; a value of 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 Dmitri Ivanovich Mendeleev (sometimes transliterated as Mendeleyev or Mendeleef) ( ; russian: links=no, Дмитрий Иванович Менделеев, tr. , ; 8 February Old_Style_and_New_Style_dates">O.S._27_January.html" ;"title="O ...

's method gave . The estimated boiling point of is also uncertain; the estimates and , as well as the extrapolation from Mendeleev's method of , 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 Electronegativity, symbolized as , 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 atomic number and the ...

, 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 Relativistic quantum chemistry combines relativistic mechanics with quantum chemistry to calculate elemental properties and structure, especially for the heavier elements of the periodic table. A prominent example is an explanation for the color of ...

, 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

Due to francium being very unstable, 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 Rubidium is the chemical element with the symbol Rb and atomic number 37. It is a very soft, whitish-grey solid in the alkali metal group, similar to potassium and caesium. Rubidium is the first alkali metal in the group to have a density higher ...

tartrate), the

chloroplatinate {{Short pages monitor. 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 Moldavia ( ro, Moldova, or , literally "The Country of Moldavia"; in Romanian Cyrillic: or ; chu, Землѧ Молдавскаѧ; el, Ἡγεμονία τῆς Μολδαβίας) is a historical region and former principality in Centr ...

, 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 The Nobel Prizes ( ; sv, Nobelpriset ; no, Nobelprisen ) are five separate prizes that, according to Alfred Nobel's will of 1895, are awarded to "those who, during the preceding year, have conferred the greatest benefit to humankind." Alfr ...

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

Eka-caesium was discovered on January 7, 1939, by Marguerite Perey of the Curie Institute in Paris, when she purified a sample of

-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 Thorium is a weakly radioactive metallic chemical element with the symbol Th and atomic number 90. Thorium is silvery and tarnishes black when it is exposed to air, forming thorium dioxide; it is moderately soft and malleable and has a high ...

, radium,

lead Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cut, ...

, bismuth, or

thallium Thallium is a chemical element with the symbol Tl and atomic number 81. It is a gray post-transition metal that is not found free in nature. When isolated, thallium resembles tin, but discolors when exposed to air. Chemists William Crookes an ...

. 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 Alpha decay or α-decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle (helium nucleus) and thereby transforms or 'decays' into a different atomic nucleus, with a mass number that is reduced by four and an at ...

of actinium-227. Perey then attempted to determine the proportion of

beta decay In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which a beta particle (fast energetic electron or positron) is emitted from an atomic nucleus, transforming the original nuclide to an isobar of that nuclide. For ...

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 The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...

(IUPAC) in 1949, becoming the second element after

gallium Gallium is a chemical element with the Symbol (chemistry), symbol Ga and atomic number 31. Discovered by France, French chemist Paul-Émile Lecoq de Boisbaudran in 1875, Gallium is in boron group, group 13 of the periodic table and is similar to ...

to be named after France. It was assigned the symbol Fa, but this abbreviation was revised to the current Fr shortly thereafter. Francium was the last element discovered in nature, rather than synthesized, following

hafnium Hafnium is a chemical element with the symbol Hf and atomic number 72. A lustrous, silvery gray, tetravalent transition metal, hafnium chemically resembles zirconium and is found in many zirconium minerals. Its existence was predicted by Dmitri M ...

and rhenium. Further research into francium's structure was carried out by, among others,

Sylvain Lieberman Sylvain is the French form of Silvanus. It may refer to: People *Sylvain Archambault, Canadian director * Sylvain Bied (1965–2011), French footballer and manager *Sylvain Cappell (born 1946), American mathematician *Sylvain Chavanel (born 1979 ...

and his team at

CERN The European Organization for Nuclear Research, known as CERN (; ; ), is an intergovernmental organization that operates the largest particle physics laboratory in the world. Established in 1954, it is based in a northwestern suburb of Gen ...

in the 1970s and 1980s.

Occurrence

²²³Fr is the result of the alpha decay of ²²⁷Ac and can be found in trace amounts in

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...

s. In a given sample of uranium, there is estimated to be only one francium atom for every 1 × 10¹⁸ uranium atoms. It is also calculated that there is a total mass of at most 30 g or, as other sources suggest, 340 to 550 g of francium in the

at any given time.

Production

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 A magneto-optical trap (MOT) is an apparatus which uses laser cooling and a spatially-varying magnetic field to create a trap which can produce samples of cold, trapped, neutral atoms. Temperatures achieved in a MOT can be as low as several microk ...

(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 Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic ta ...

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 Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...

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.

Notes

References

External links

Francium
at '' The Periodic Table of Videos'' (University of Nottingham)
WebElements.com – Francium

* Scerri, Eric (2013). ''A Tale of Seven Elements'', Oxford University Press, Oxford, {{Subject bar , portal=Chemistry , book1=Francium , book2=Period 7 elements , book3=Alkali metals , book4=Chemical elements (sorted alphabetically) , book5=Chemical elements (sorted by number) , Book6=Alkali metals , commons=y , wikt=y , wikt-search=francium , v=y , v-search=Francium atom Chemical elements Alkali metals Eponyms Science and technology in France Chemical elements with body-centered cubic structure Chemical elements predicted by Dmitri Mendeleev