Francium

Francium is a chemical element with symbol Fr and atomic number 87. It used to be known 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.

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]

Francium,  87Fr
Francium
Pronunciation/ˈfrænsiəm/ (FRAN-see-əm)
Mass number223 (most stable isotope)
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
Groupgroup 1 (alkali metals)
Periodperiod 7
Blocks-block
Element category  alkali metal
Electron configuration[Rn] 7s1
Electrons per shell
2, 8, 18, 32, 18, 8, 1
Physical properties
Phase at STPsolid (predicted)
Melting point? 300 K ​(30 °C, ​80 °F)
Boiling point? 950 K ​(680 °C, ​1300 °F)
Density (near r.t.)2.8–3.0 g/cm3 (extrapolated)[1]
Vapor pressure (extrapolated)
P (Pa) 1 10 100 1 k 10 k 100 k
at T (K) 404 454 519 608 738 946
Atomic properties
Oxidation states+1 (a strongly basic oxide)
ElectronegativityPauling scale: >0.79
Ionization energies
  • 1st: 393 kJ/mol[2]
Covalent radius260 pm (extrapolated)
Van der Waals radius348 pm (extrapolated)
Other properties
Natural occurrencefrom decay
Crystal structurebody-centered cubic (bcc)
Body-centered cubic crystal structure for francium

(extrapolated)
Thermal conductivity15 W/(m·K) (extrapolated)
Electrical resistivity3 µΩ·m (calculated)
Magnetic orderingParamagnetic
CAS Number7440-73-5
History
Namingafter France, homeland of the discoverer
Discovery and first isolationMarguerite Perey (1939)
Main isotopes of francium
Iso­tope Abun­dance Half-life (t1/2) Decay mode Pro­duct
221Fr trace 4.8 min α 217At
222Fr syn 14.2 min β 222Ra
223Fr trace 22.00 min β 223Ra
α 219At

Characteristics

Elektronskal 87
Electron configuration. The letter on each electron indicates the shell to which it belongs.

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 calculated to be around 27 °C (80 °F, 300 K).[9] The melting point is uncertain because of the element's extreme rarity and radioactivity. The estimated boiling point of 677 °C (1250 °F, 950 K) is also uncertain.

Linus Pauling estimated the electronegativity of francium at 0.7 on the Pauling scale, the same as caesium;[10] 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.[11] Francium has a slightly higher ionization energy than caesium,[12] 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.[13] 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.[13]

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 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.[14][15] Nearly all francium salts are water-soluble.[16]

Isotopes

There are 34 known isotopes of francium ranging in atomic mass from 199 to 232.[17] 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.[18]

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.[19] Francium-223 is the fifth product of the actinium decay series as the daughter isotope of actinium-227.[20] 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).[21]

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.[20] 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.[22]

Applications

Due to its instability and rarity, there are no commercial applications for francium.[23][24][25][20] It has been used for research purposes in the fields of chemistry[26] 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.[24]

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.[27] 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.[28]

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.[29] 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).[30] 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.[31] Shortly thereafter, Dobroserdov began to focus on his teaching career at the Polytechnic Institute of Odessa, and he did not pursue the element further.[30]

The following year, English chemists Gerald J. F. Druce and Frederick H. Loring analyzed X-ray photographs of manganese(II) sulfate.[31] 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.[30]

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.[31][32] In 1934, H.G. MacPherson of UC Berkeley disproved the effectiveness of Allison's device and the validity of his discovery.[33]

In 1936, Romanian physicist Horia Hulubei and his French colleague Yvette Cauchois also analyzed pollucite, this time using their high-resolution X-ray apparatus.[30] 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.[31] 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.[30] All other previous purported discoveries of element 87 were ruled out due to francium's very limited half-life.[31]

Perey's analysis

Eka-caesium was discovered in on 7 January 1939 by Marguerite Perey of the Curie Institute in Paris,[34] 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.[29] 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%.[19]

Perey named the new isotope actinium-K (it is now referred to as francium-223)[29] 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.[29] 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.[35] Francium was the last element discovered in nature, rather than synthesized, following hafnium and rhenium.[29] Further research into francium's structure was carried out by, among others, Sylvain Lieberman and his team at CERN in the 1970s and 1980s.[36]

Occurrence

Pichblende
This sample of uraninite contains about 100,000 atoms (3.3×1020 g) of francium-223 at any given time.[24]

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.[24] It is also calculated that there is at most 30 g of francium in the Earth's crust at any given time.[37]

Production

Franciumtrap
A magneto-optical trap, which can hold neutral francium atoms for short periods of time.[38]
Francium
Image of light emitted by a sample of 200,000 francium atoms in a magneto-optical trap
Fr,87
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 in the physics department at the State University of New York at Stony Brook in 1995.[39] 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.[38] 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.[38] 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.[40][41]

Other synthesis methods include bombarding radium with neutrons, and bombarding thorium with protons, deuterons, or helium ions.[19]

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.[42]

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][9][24]

Notes

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

References

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  35. ^ Grant, Julius (1969). "Francium". Hackh's Chemical Dictionary. McGraw-Hill. pp. 279–280. ISBN 978-0-07-024067-4.
  36. ^ "History". Francium. State University of New York at Stony Brook. February 20, 2007. Archived from the original on February 3, 1999. Retrieved March 26, 2007.
  37. ^ Winter, Mark. "Geological information". Francium. The University of Sheffield. Retrieved March 26, 2007.
  38. ^ a b c "Cooling and Trapping". Francium. State University of New York at Stony Brook. February 20, 2007. Archived from the original on November 22, 2007. Retrieved May 1, 2007.
  39. ^ "Production of Francium". Francium. State University of New York at Stony Brook. February 20, 2007. Archived from the original on October 12, 2007. Retrieved March 26, 2007.
  40. ^ Orozco, Luis A. (September 30, 2014). "Project Closeout Report: Francium Trapping Facility at TRIUMF". U.S. Department of Energy. doi:10.2172/1214938.
  41. ^ Tandecki, M; Zhang, J.; Collister, R.; Aubin, S.; Behr, J. A.; Gomez, E.; Gwinner, G.; Orozco, L. A.; Pearson, M. R. (2013). "Commissioning of the Francium Trapping Facility at TRIUMF". Journal of Instrumentation. 8 (12): P12006. arXiv:1312.3562. doi:10.1088/1748-0221/8/12/P12006.
  42. ^ Keller, Cornelius; Wolf, Walter; Shani, Jashovam, "Radionuclides, 2. Radioactive Elements and Artificial Radionuclides", Ullmann's Encyclopedia of Industrial Chemistry, Weinheim: Wiley-VCH, doi:10.1002/14356007.o22_o15

External links

Alkali metal

The alkali metals are a group (column) in the periodic table consisting of the chemical elements lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr). This group lies in the s-block of the periodic table of elements as 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.

The alkali metals are all shiny, soft, highly reactive metals at standard temperature and pressure and readily lose their outermost electron to form cations with charge +1. They can all be cut easily with a knife due to their softness, exposing a shiny surface that tarnishes rapidly in air due to oxidation by atmospheric moisture and oxygen (and in the case of lithium, nitrogen). Because of their high reactivity, they must be stored under oil to prevent reaction with air, and are found naturally only in salts and never as the free elements. Caesium, the fifth alkali metal, is the most reactive of all the metals. In the modern IUPAC nomenclature, the alkali metals comprise the group 1 elements, excluding hydrogen (H), which is nominally a group 1 element but not normally considered to be an alkali metal as it rarely exhibits behaviour comparable to that of the alkali metals. All the alkali metals react with water, with the heavier alkali metals reacting more vigorously than the lighter ones.

All of the discovered alkali metals occur in nature as their compounds: in order of abundance, sodium is the most abundant, followed by potassium, lithium, rubidium, caesium, and finally francium, which is very rare due to its extremely high radioactivity; francium occurs only in the minutest traces in nature as an intermediate step in some obscure side branches of the natural decay chains. Experiments have been conducted to attempt the synthesis of ununennium (Uue), which is likely to be the next member of the group, but they have all met with failure. However, ununennium may not be an alkali metal due to relativistic effects, which are predicted to have a large influence on the chemical properties of superheavy elements; even if it does turn out to be an alkali metal, it is predicted to have some differences in physical and chemical properties from its lighter homologues.

Most alkali metals have many different applications. One of the best-known applications of the pure elements is the use of rubidium and caesium in atomic clocks, of which caesium atomic clocks are the most accurate and precise representation of time. A common application of the compounds of sodium is the sodium-vapour lamp, which emits light very efficiently. Table salt, or sodium chloride, has been used since antiquity. Lithium finds use as a psychiatric medication. Sodium and potassium are also essential elements, having major biological roles as electrolytes, and although the other alkali metals are not essential, they also have various effects on the body, both beneficial and harmful.

Caesium perchlorate

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.

CsClO4 is the least soluble of the alkali metal perchlorates (followed by Rb, K, Li, and Na), a property which may be used for separatory purposes and even for gravimetric analysis. This low solubility played an important role in the characterization of francium as an alkali metal, as francium perchlorate coprecipitates with caesium perchlorate.

When heated, CsClO4 decomposes to caesium chloride above 250 °C. Like all perchlorates, it is a strong oxidant and may react violently with reducing agents and organic materials, especially at elevated temperatures.

Coprecipitation

In chemistry, coprecipitation (CPT) or co-precipitation is the carrying down by a precipitate of substances normally soluble under the conditions employed. Analogously, in medicine, coprecipitation is specifically the precipitation of an unbound "antigen along with an antigen-antibody complex".Coprecipitation is an important issue in chemical analysis, where it is often undesirable, but in some cases it can be exploited. In gravimetric analysis, which consists on precipitating the analyte and measuring its mass to determine its concentration or purity, coprecipitation is a problem because undesired impurities often coprecipitate with the analyte, resulting in excess mass. This problem can often be mitigated by "digestion" (waiting for the precipitate to equilibrate and form larger, purer particles) or by redissolving the sample and precipitating it again.

On the other hand, in the analysis of trace elements, as is often the case in radiochemistry, coprecipitation is often the only way of separating an element. Since the trace element is too dilute (sometimes less than a part per trillion) to precipitate by conventional means, it is typically coprecipitated with a carrier, a substance that has a similar crystalline structure that can incorporate the desired element. An example is the separation of francium from other radioactive elements by coprecipitating it with caesium salts such as caesium perchlorate. Otto Hahn is credited for promoting the use of coprecipitation in radiochemistry.

There are three main mechanisms of coprecipitation: inclusion, occlusion, and adsorption. An inclusion occurs when the impurity occupies a lattice site in the crystal structure of the carrier, resulting in a crystallographic defect; this can happen when the ionic radius and charge of the impurity are similar to those of the carrier. An adsorbate is an impurity that is weakly bound (adsorbed) to the surface of the precipitate. An occlusion occurs when an adsorbed impurity gets physically trapped inside the crystal as it grows.

Besides its applications in chemical analysis and in radiochemistry, coprecipitation is also "potentially important to many environmental issues closely related to water resources, including acid mine drainage, radionuclide migration in fouled waste repositories, metal contaminant transport at industrial and defense sites, metal concentrations in aquatic systems, and wastewater treatment technology".Coprecipitation is also used as a method of magnetic nanoparticle synthesis.

Dilithium (Star Trek)

In the Star Trek fictional universe, dilithium is an invented material which serves as a controlling agent in the faster-than-light warp drive. In the original series, dilithium crystals were rare and could not be replicated, making the search for them a recurring plot element. According to a periodic table shown during a Next Generation episode, it has the chemical symbol Dt and the atomic number 87, which in reality belongs to francium.In the real world, dilithium (Li2) is a molecule composed of two covalently bonded lithium atoms.

Ditungsten tetra(hpp)

Tetrakis(hexahydropyrimidinopyrimidine)ditungsten(II), known as ditungsten tetra(hpp), is the name of the coordination compound with the formula W2(hpp)4. This material consists of a pair of tungsten centers linked by the conjugate base of four hexahydropyrimidopyrimidine (hpp) ligands. It adopts a structure sometimes called a Chinese lantern structure or paddlewheel compound, the prototype being copper(II) acetate.

The molecule is of research interest because it has the lowest ionization energy (3.51 eV) of all stable chemical elements or chemical compounds as of the year 2005. This value is even lower than of caesium with 3.89 eV (or 375 kJ/mol) located at the extreme left lower corner of the periodic table (although francium is at a lower position in the periodic table compared to caesium, it has a higher ionization energy and is radioactive) or known metallocene reducing agents such as decamethylcobaltocene with 4.71 eV.

Electronegativity

Electronegativity, symbol χ, is a chemical property that describes the tendency of an atom to attract a shared pair of electrons (or electron density) 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.

On the most basic level, electronegativity is determined by factors like the nuclear charge (the more protons an atom has, the more "pull" it will have on electrons) and the number/location of other electrons present in the atomic shells (the more electrons an atom has, the farther from the nucleus the valence electrons will be, and as a result the less positive charge they will experience—both because of their increased distance from the nucleus, and because the other electrons in the lower energy core orbitals will act to shield the valence electrons from the positively charged nucleus).

The opposite of electronegativity is electropositivity: a measure of an element's ability to donate electrons.

The term "electronegativity" was introduced by Jöns Jacob Berzelius in 1811,

though the concept was known even before that and was studied by many chemists including Avogadro.

In spite of its long history, an accurate scale of electronegativity was not developed until 1932, when Linus Pauling proposed an electronegativity scale, which depends on bond energies, as a development of valence bond theory. It has been shown to correlate with a number of other chemical properties. Electronegativity cannot be directly measured and must be calculated from other atomic or molecular properties. Several methods of calculation have been proposed, and although there may be small differences in the numerical values of the electronegativity, all methods show the same periodic trends between elements.

The most commonly used method of calculation is that originally proposed by Linus Pauling. This gives a dimensionless quantity, commonly referred to as the Pauling scale (χr), on a relative scale running from around 0.7 to 3.98 (hydrogen = 2.20). When other methods of calculation are used, it is conventional (although not obligatory) to quote the results on a scale that covers the same range of numerical values: this is known as an electronegativity in Pauling units.

As it is usually calculated, electronegativity is not a property of an atom alone, but rather a property of an atom in a molecule. Properties of a free atom include ionization energy and electron affinity. It is to be expected that the electronegativity of an element will vary with its chemical environment, but it is usually considered to be a transferable property, that is to say that similar values will be valid in a variety of situations.

Caesium is the least electronegative element in the periodic table (=0.79), while fluorine is most electronegative (=3.98). Francium and caesium were originally both assigned 0.7; caesium's value was later refined to 0.79, but no experimental data allows a similar refinement for francium. However, francium's ionization energy is known to be slightly higher than caesium's, in accordance with the relativistic stabilization of the 7s orbital, and this in turn implies that francium is in fact more electronegative than caesium.

Inorganic compounds by element

This is a list of common inorganic and organometallic compounds of each element. Compounds are listed alphabetically by their chemical element name rather than by symbol, as in list of inorganic compounds.

Isotopes of francium

Francium (87Fr) has no stable isotopes. A standard atomic weight cannot be given. Its most stable isotope is 223Fr with a half-life of 22 minutes, occurring in trace quantities as an intermediate decay product of 235U.

Of elements whose most stable isotopes have been identified with certainty, francium is the most unstable. All elements with atomic number of greater than or equal to 106 (seaborgium) have most-stable-known isotopes shorter than that of francium, but those elements have only a relatively small number of isotopes discovered, thus, there is the possibility of a yet-unknown isotope having a longer half-life.

Liquid metal

Liquid metal consists of alloys with very low melting points which form a eutectic that is liquid at room temperature. The standard metal used to be mercury, but gallium-based alloys, which are lower both in their vapor pressure at room temperature and toxicity, are being used as a replacement in various applications.A few elemental metals are liquid at or near room temperature. The most well known is mercury(Hg), which is molten above −38.8 °C (234.3 K, −37.9 °F). Others include caesium(Cs), which has a melting point of 28.5 °C (83.3 °F), rubidium (Rb)(39 °C [102 °F]), francium (Fr)(estimated at 27 °C [81 °F]), and gallium (Ga)(30 °C [86 °F]).

Major actinide

Major actinides is a term used in the nuclear power industry that refers to the plutonium and uranium present in used nuclear fuel, as opposed to the minor actinides neptunium, americium, curium, berkelium, and californium.

Marguerite Perey

Marguerite Catherine Perey (19 October 1909 – 13 May 1975) 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.

Period 7 element

A period 7 element is one of the chemical elements in the seventh row (or period) 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 plutonium.

Periodic table (crystal structure)

For elements that are solid at standard temperature and pressure the table gives the crystalline structure of the most thermodynamically stable form(s) in those conditions. In all other cases the structure given is for the element at its melting point. Data is presented only for the first 114 elements as well as the 118th (hydrogen through flerovium and oganesson), and predictions are given for elements that have never been produced in bulk (astatine, francium, and elements 100–114 and 118).

Symbol (chemistry)

In relation to the chemical elements, a symbol is a code for a chemical element. Many functional groups have their own chemical symbol, e.g. Ph for the phenyl group, and Me for the methyl group. Chemical symbols for elements normally consist of one or two letters from the Latin alphabet, but can contain three when the element has a systematic temporary name (as of March 2017, no discovered elements have such a name), and are written with the first letter capitalized.

Earlier chemical element symbols stem from classical Latin and Greek vocabulary. For some elements, this is because the material was known in ancient times, while for others, the name is a more recent invention. For example, "He" is the symbol for helium (New Latin name, not known in ancient Roman times), "Pb" for lead (plumbum in Latin), and "Hg" for mercury (hydrargyrum in Greek). Some symbols come from other sources, like "W" for tungsten (Wolfram in German, not known in Roman times).

Temporary symbols assigned to newly or not-yet synthesized elements use 3-letter symbols based on their atomic numbers. For example, "Uno" was the temporary symbol for hassium (element 108) which had the temporary name of unniloctium.

Chemical symbols may be modified by the use of prepended superscripts or subscripts to specify a particular isotope of an atom. Additionally, appended superscripts may be used to indicate the ionization or oxidation state of an element. They are widely used in chemistry and they have been officially chosen by the International Union of Pure and Applied Chemistry (IUPAC). There are also some historical symbols that are no longer officially used.

Attached subscripts or superscripts specifying a nuclide or molecule have the following meanings and positions:

The nucleon number (mass number) is shown in the left superscript position (e.g., 14N). This number defines the specific isotope. Various letters, such as "m" and "f" may also be used here to indicate a nuclear isomer (e.g., 99mTc). Alternately, the number here can represent a specific spin state (e.g., 1O2). These details can be omitted if not relevant in a certain context.

The proton number (atomic number) may be indicated in the left subscript position (e.g., 64Gd). The atomic number is redundant to the chemical element, but is sometimes used to emphasize the change of numbers of nucleons in a nuclear reaction.

If necessary, a state of ionization or an excited state may be indicated in the right superscript position (e.g., state of ionization Ca2+).

The number of atoms of an element in a molecule or chemical compound is shown in the right subscript position (e.g., N2 or Fe2O3). If this number is one, it is normally omitted - the number one is then implicit.

A radical is indicated by a dot on the right side (e.g., Cl• for a neutral chlorine atom). This is often omitted unless relevant to a certain context because it is already deducible from the charge and atomic number information values.In Chinese, each chemical element has a dedicated character, usually created for the purpose (see Chemical elements in East Asian languages). However, Latin symbols are also used, especially in formulas.

A list of current, dated, as well as proposed and historical signs and symbols is included here with its signification. Also given is each element's atomic number, atomic weight or the atomic mass of the most stable isotope, group and period numbers on the periodic table, and etymology of the symbol.

Hazard pictographs are another type of symbols used in chemistry.

Trace radioisotope

A trace radioisotope is a radioisotope that occurs naturally in trace amounts (i.e. extremely small). Generally speaking, trace radioisotopes have half-lives that are short in comparison with the age of the Earth, since primordial nuclides tend to occur in larger than trace amounts. Trace radioisotopes are therefore present only because they are continually produced on Earth by natural processes. Natural processes which produce trace radioisotopes include cosmic ray bombardment of stable nuclides, ordinary alpha and beta decay of the long-lived heavy nuclides, thorium-232, uranium-238, and uranium-235, spontaneous fission of uranium-238, and nuclear transmutation reactions induced by natural radioactivity, such as the production of plutonium-239 and uranium-236 from neutron capture by natural uranium.

Ununennium

Ununennium, also known as eka-francium or simply 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, until 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.

Experiments aimed at the synthesis of ununennium began in December 2017 at RIKEN in Japan; another attempt by the team at the Joint Institute for Nuclear Research at Dubna, Russia is scheduled to begin in 2019. Prior to this, two unsuccessful attempts had been made to synthesize ununennium, one by an American team and one by a German team. Theoretical and experimental evidence has shown that the synthesis of ununennium would likely be far more difficult than that of the previous elements, and it may even be one of the last two elements (with unbinilium) that can be synthesized with current technology.

Ununennium's position as the seventh alkali metal suggests that it would have similar properties to its lighter congeners: lithium, sodium, potassium, rubidium, caesium, and francium; however, relativistic effects may cause some of its properties to differ from those expected from a straight application of periodic trends. For example, ununennium is expected to be less reactive than caesium and francium and to be closer in behavior to potassium or rubidium, and while it should show the characteristic +1 oxidation state of the alkali metals, it is also predicted to show the +3 oxidation state, which is unknown in any other alkali metal.

Uraninite

Uraninite, formerly pitchblende, is a radioactive, uranium-rich mineral and ore with a chemical composition that is largely UO2, but due to oxidation the mineral typically contains variable proportions of U3O8. Additionally, due to radioactive decay, the ore also contains oxides of lead and trace amounts of helium. It may also contain thorium and rare earth elements.

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