Francium

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Francium, 87Fr
Francium
Pronunciation /ˈfrænsiəm/ (FRAN-see-əm)
Mass number [223]
Francium in the periodic table
Hydrogen Helium
Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon
Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon
Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton
Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon
Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon
Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson
Cs

Fr

(Uue)
radonfranciumradium
Atomic number (Z)87
Group group 1: H and alkali metals
Period period 7
Block s-block
Element category   Alkali metal
Electron configuration [ Rn ] 7s1
Electrons per shell
2, 8, 18, 32, 18, 8, 1
Physical properties
Phase at  STP solid at 0 °C, liquid at r.t.
Melting point 281.0  K (8.0 °C,46.4 °F)(estimated) [1]
Boiling point 890 K(620 °C,1150 °F)(estimated) [1]
Density (near r.t.)2.48 g/cm3(estimated) [1]
Vapor pressure (extrapolated)
P (Pa)1101001 k10 k100 k
at T (K)404454519608738946
Atomic properties
Oxidation states +1 (a strongly basic oxide)
Electronegativity Pauling scale: >0.79
Ionization energies
  • 1st: 393 kJ/mol [2]
Covalent radius 260  pm (extrapolated)
Van der Waals radius 348 pm(extrapolated)
Other properties
Natural occurrence from decay
Crystal structure body-centered cubic (bcc)
Cubic-body-centered.svg

(extrapolated)
Thermal conductivity 15 W/(m·K)(extrapolated)
Electrical resistivity 3 µΩ·m(calculated)
Magnetic ordering Paramagnetic
CAS Number 7440-73-5
History
Namingafter France, homeland of the discoverer
Discovery and first isolation Marguerite Perey (1939)
Main isotopes of francium
Iso­tope Abun­dance Half-life (t1/2) Decay mode Pro­duct
212Fr syn 20.0 minβ+ 212Rn
α 208At
221Fr trace 4.8 min α 217At
222Fr syn 14.2 min β 222Ra
223Frtrace22.00 minβ 223Ra
α 219At
| references

Francium is a chemical element with the symbol  Fr and atomic number  87. Prior to its discovery, it was referred to as eka-caesium. It is extremely radioactive; its most stable isotope, francium-223 (originally called actinium 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, and is the second rarest naturally occurring element (after astatine). The isotopes of francium decay quickly into astatine, radium, and radon. The electronic structure of a francium atom is [Rn] 7s1, and so the element is classed as an alkali metal.

Contents

Bulk francium has never been viewed. Because of the general appearance of the other elements in its periodic table column, it is assumed 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 caused by 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. [3] It was the last element first discovered in nature, rather than by synthesis. [note 1] Outside the laboratory, francium is extremely rare, with trace amounts found in uranium and thorium ores, where the isotope francium-223 continually forms and decays. As little as 20–30 g (one ounce) exists at any given time throughout the Earth's crust; the other isotopes (except for francium-221) are entirely synthetic. The largest amount produced in the laboratory was a cluster of more than 300,000 atoms. [4]

Characteristics

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. [5] All isotopes of francium decay into astatine, radium, or radon. [5] Francium-223 also has a shorter half-life than the longest-lived isotope of each synthetic element up to and including element 105, dubnium. [6]

Francium is an alkali metal whose chemical properties mostly resemble those of caesium. [6] A heavy element with a single valence electron, [7] it has the highest equivalent weight of any element. [6] Liquid francium—if created—should have a surface tension of 0.05092  N/m at its melting point. [8] Francium's melting point was estimated to be around 8.0 °C (46.4 °F, 281.0 K). [1] 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±1.5 °C (68±2.7 °F, 293±1.5 K). The estimated boiling point of 620 °C (1150 °F, 890 K) is also uncertain; the estimate 598 °C (1108 °F, 871 K), as well as the extrapolation from Mendeleev's method of 640 °C (1180 °F, 910 K), have also been suggested. [1] [8] The density of francium is expected to be around 2.48 g/cm3 (Mendeleev's method extrapolates 2.4 g/cm3). [1]

Linus Pauling estimated the electronegativity of francium at 0.7 on the Pauling scale, the same as caesium; [9] 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. [10] Francium has a slightly higher ionization energy than caesium, [11] 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. [12] The CsFr molecule is predicted to have 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. [12]

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 provides other methods of separation. [13] [14] Nearly all francium salts are water-soluble. [15]

Isotopes

There are 34 known isotopes of francium ranging in atomic mass from 199 to 232. [16] Francium has seven metastable nuclear isomers. [6] Francium-223 and francium-221 are the only isotopes that occur in nature, though the former is far more common. [17]

Francium-223 is the most stable isotope, with a half-life of 21.8 minutes, [6] and it is highly unlikely that an isotope of francium with a longer half-life will ever be discovered or synthesized. [18] Francium-223 is the fifth product of the actinium decay series as the daughter isotope of actinium-227. [19] 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). [20]

Francium-221 has a half-life of 4.8 minutes. [6] It is the ninth product of the neptunium decay series as a daughter isotope of actinium-225. [19] Francium-221 then decays into astatine-217 by alpha decay (6.457 MeV decay energy). [6]

The least stable ground state isotope is francium-215, with a half-life of 0.12 μs: it undergoes a 9.54 MeV alpha decay to astatine-211. [6] Its metastable isomer, francium-215m, is less stable still, with a half-life of only 3.5 ns. [21]

Applications

Due to its instability and rarity, there are no commercial applications for francium. [22] [23] [24] [19] It has been used for research purposes in the fields of chemistry [25] and of atomic structure. Its use as a potential diagnostic aid for various cancers has also been explored, [5] but this application has been deemed impractical. [23]

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. [26] 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. [27]

History

As early as 1870, chemists thought that there should be an alkali metal beyond caesium, with an atomic number of 87. [5] It was then referred to by the provisional name eka-caesium . [28] Research teams attempted to locate and isolate this missing element, and at least four false claims were made that the element had been found before an authentic discovery was made.

Erroneous and incomplete discoveries

Soviet chemist D. K. 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). [29] 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. [30] Shortly thereafter, Dobroserdov began to focus on his teaching career at the Polytechnic Institute of Odessa, and he did not pursue the element further. [29]

The following year, English chemists Gerald J. F. Druce and Frederick H. Loring analyzed X-ray photographs of manganese(II) sulfate. [30] 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. [29]

In 1930, Fred Allison of the Alabama Polytechnic Institute claimed to have discovered element 87 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. [30] [31] In 1934, H.G. MacPherson of UC Berkeley disproved the effectiveness of Allison's device and the validity of his discovery. [32]

In 1936, Romanian physicist Horia Hulubei and his French colleague Yvette Cauchois also analyzed pollucite, this time using their high-resolution X-ray apparatus. [29] 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. [30] 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. [29] All other previous purported discoveries of element 87 were ruled out due to francium's very limited half-life. [30]

Perey's analysis

Eka-caesium was discovered in on 7 January 1939 by Marguerite Perey of the Curie Institute in Paris, [33] 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. [28] 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%. [18]

Perey named the new isotope actinium-K (it is now referred to as francium-223) [28] 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. [28] Perey then suggested francium, after France. This name was officially adopted by the International Union of Pure and Applied Chemistry (IUPAC) in 1949, [5] becoming the second element after gallium to be named after France. It was assigned the symbol Fa, but this abbreviation was revised to the current Fr shortly thereafter. [34] Francium was the last element discovered in nature, rather than synthesized, following hafnium and rhenium. [28] Further research into francium's structure was carried out by, among others, Sylvain Lieberman and his team at CERN in the 1970s and 1980s. [35]

Occurrence

This sample of uraninite contains about 100,000 atoms (3.3x10 g) of francium-223 at any given time. Pichblende.jpg
This sample of uraninite contains about 100,000 atoms (3.3×10 g) of francium-223 at any given time.

223Fr is the result of the alpha decay of 227Ac and can be found in trace amounts in uranium minerals. [6] In a given sample of uranium, there is estimated to be only one francium atom for every 1 × 1018 uranium atoms. [23] It is also calculated that there is a total mass of at most 30 g of francium in the Earth's crust at any given time. [36]

Production

A magneto-optical trap, which can hold neutral francium atoms for short periods of time. Franciumtrap.PNG
A magneto-optical trap, which can hold neutral francium atoms for short periods of time.
Francium.jpg
Image of light emitted by a sample of 200,000 francium atoms in a magneto-optical trap
Fr,87.jpg
Heat image of 300,000 francium atoms in a magneto-optical trap

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. [38] 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. [37] 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. [37] The original apparatus could trap up to a few thousand atoms, while a later improved design could trap over 300,000 at a time. [4] 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. [39] [40]

Other synthesis methods include bombarding radium with neutrons, and bombarding thorium with protons, deuterons, or helium ions. [18]

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. [41]

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. [4] Francium has not been synthesized in amounts large enough to weigh. [5] [23] [42]

See also

Notes

  1. Some synthetic elements, like technetium and plutonium, have later been found in nature.

Related Research Articles

Alkali metal Group of highly-reactive chemical elements

The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr). Together with hydrogen they comprise group 1, which lies in the s-block of the periodic table. All alkali metals have their outermost electron in an s-orbital: this shared electron configuration results in their having very similar characteristic properties. Indeed, the alkali metals provide the best example of group trends in properties in the periodic table, with elements exhibiting well-characterised homologous behaviour. This family of elements is also known as the lithium family after its leading element.

Actinium Chemical element with atomic number 89

Actinium is a chemical element with the symbol Ac and atomic number 89. It was first isolated by French chemist André-Louis Debierne in 1899. Friedrich Oskar Giesel later independently isolated it in 1902 and, unaware that it was already known, gave it the name emanium. Actinium gave the name to the actinide series, a group of 15 similar elements between actinium and lawrencium in the periodic table. It is also sometimes considered the first of the 7th-period transition metals, although lawrencium is less commonly given that position. Together with polonium, radium, and radon, actinium was one of the first non-primordial radioactive elements to be isolated.

Astatine Chemical element with atomic number 85

Astatine is a radioactive 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-lived; the most stable is astatine-210, with a half-life of 8.1 hours. A sample of the pure element has never been assembled, because any macroscopic specimen would be immediately vaporized by the heat of its own radioactivity.

Chemical element a species of atoms having the same number of protons in the atomic nucleus

A chemical element is a species of atom having the same number of protons in their atomic nuclei. For example, the atomic number of oxygen is 8, so the element oxygen consists of all atoms which have 8 protons.

Electronegativity, symbol χ, is a chemical property that describes the tendency of an atom to attract a shared pair of electrons towards itself. An atom's electronegativity is affected by both its atomic number and the distance at which its valence electrons reside from the charged nucleus. The higher the associated electronegativity number, the more an atom or a substituent group attracts electrons towards itself.

Radium Chemical element with atomic number 88

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 (rather than oxygen) on exposure to air, forming a black surface layer of radium nitride (Ra3N2). All isotopes of radium are highly radioactive, with the most stable isotope being radium-226, which has a half-life of 1600 years and decays into radon gas (specifically the isotope radon-222). When radium decays, ionizing radiation is a product, which can excite fluorescent chemicals and cause radioluminescence.

Symbol (chemistry) an arbitrary or conventional sign used in chemical science to represent a chemical element

In chemistry, a symbol is an abbreviation for a chemical element. Symbols for chemical elements normally consist of one or two letters from the Latin alphabet and are written with the first letter capitalised.

Ununennium Chemical element with atomic number 119

Ununennium, also known as eka-francium or element 119, is the hypothetical chemical element with symbol Uue and atomic number 119. Ununennium and Uue are the temporary systematic IUPAC name and symbol respectively, which are used until the element is discovered, confirmed, and a permanent name is decided upon. In the periodic table of the elements, it is expected to be an s-block element, an alkali metal, and the first element in the eighth period. It is the lightest element that has not yet been synthesized.

Tennessine is a synthetic chemical element with the symbol Ts and atomic number 117. It is the second-heaviest known element and the penultimate element of the 7th period of the periodic table.

An extended periodic table theorizes about chemical elements beyond those currently known in the periodic table and proven up through oganesson, which completes the seventh period (row) in the periodic table at atomic number (Z) 118.

Mendeleevs predicted elements elements predicted to exist but not yet found on the first periodic table

Dmitri Mendeleev published a periodic table of the chemical elements in 1869 based on properties that appeared with some regularity as he laid out the elements from lightest to heaviest. When Mendeleev proposed his periodic table, he noted gaps in the table and predicted that as-then-unknown elements existed with properties appropriate to fill those gaps. He named them eka-boron, eka-aluminium and eka-silicon, with respective atomic masses of 44, 68, and 72.

A period 6 element is one of the chemical elements in the sixth row (or period) of the periodic table of the elements, including the lanthanides. The periodic table is laid out in rows to illustrate recurring (periodic) trends in the chemical behaviour of the elements as their atomic number increases: a new row is begun when chemical behaviour begins to repeat, meaning that elements with similar behaviour fall into the same vertical columns. The sixth period contains 32 elements, tied for the most with period 7, beginning with caesium and ending with radon. Lead is currently the last stable element; all subsequent elements are radioactive. For bismuth, however, its only primordial isotope, 209Bi, has a half-life of more than 1019 years, over a billion times longer than the current age of the universe. As a rule, period 6 elements fill their 6s shells first, then their 4f, 5d, and 6p shells, in that order; however, there are exceptions, such as gold.

Decay chain series of elements in radioactive decay

In nuclear science, the decay chain refers to a series of radioactive decays of different radioactive decay products as a sequential series of transformations. It is also known as a "radioactive cascade". Most radioisotopes do not decay directly to a stable state, but rather undergo a series of decays until eventually a stable isotope is reached.

A period 7 element is one of the chemical elements in the seventh row of the periodic table of the chemical elements. The periodic table is laid out in rows to illustrate recurring (periodic) trends in the chemical behaviour of the elements as their atomic number increases: a new row is begun when chemical behaviour begins to repeat, meaning that elements with similar behaviour fall into the same vertical columns. The seventh period contains 32 elements, tied for the most with period 6, beginning with francium and ending with oganesson, the heaviest element currently discovered. As a rule, period 7 elements fill their 7s shells first, then their 5f, 6d, and 7p shells, in that order; however, there are exceptions, such as uranium.

Marguerite Perey 20th-century French physicist

Marguerite Catherine Perey was a French physicist and a student of Marie Curie. In 1939, Perey discovered the element francium by purifying samples of lanthanum that contained actinium. In 1962, she was the first woman to be elected to the French Académie des Sciences, an honor denied to her mentor Curie. Perey died of cancer in 1975.

Actinium (89Ac) has no stable isotopes and no characteristic terrestrial isotopic composition, thus a standard atomic weight cannot be given. There are 32 known isotopes, from 205Ac to 236Ac, and 7 isomers. Three isotopes are found in nature, 225Ac, 227Ac and 228Ac, as intermediate decay products of, respectively, 237Np, 235U, and 232Th. 228Ac and 225Ac are extremely rare, so almost all natural actinium is 227Ac.

Astatine (85At) has 39 known isotopes, all of which are radioactive; the range of their mass numbers is from 191 to 229. There also exist 23 metastable excited states. The longest-lived isotope is 210At, which has a half-life of 8.1 hours; the longest-lived isotope existing in naturally occurring decay chains is 219At with a half-life of 56 seconds.

Caesium perchlorate chemical compound

Caesium perchlorate or cesium perchlorate (CsClO4), is a perchlorate of caesium. It forms white crystals, which are sparingly soluble in cold water and ethanol. It dissolves more easily in hot water.

Radon-222 is the most stable isotope of radon, with a half-life of approximately 3.8 days. It is transient in the decay chain of primordial uranium-238 and is the immediate decay product of radium-226. Radon-222 was first observed in 1899, and was identified as an isotope of a new element several years later. In 1957, the name radon, formerly the name of only radon-222, became the name of the element. Owing to its gaseous nature and high radioactivity, radon-222 is one of the leading causes of lung cancer.

Unbiunium, also known as eka-actinium or simply element 121, is the hypothetical chemical element with symbol Ubu and atomic number 121. Unbiunium and Ubu are the temporary systematic IUPAC name and symbol respectively, which are used until the element is discovered, confirmed, and a permanent name is decided upon. In the periodic table of the elements, it is expected to be the first of the superactinides, and the third element in the eighth period: analogously to lanthanum and actinium, it could be considered the fifth member of group 3 and the first member of the fifth-row transition metals. It has attracted attention because of some predictions that it may be in the island of stability, although newer calculations expect the island to actually occur at a slightly lower atomic number, closer to copernicium and flerovium.

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