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Aperiod 6 element is one of thechemical elements in the sixth row (orperiod) of theperiodic table of the chemical elements, including thelanthanides. 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 withperiod 7, beginning withcaesium and ending withradon.Lead is currently the last stable element; all subsequent elements areradioactive. Forbismuth, however, its onlyprimordial isotope,209Bi, has a half-life of more than 1019 years, over a billion times longer than the currentage of the universe. As a rule, period 6 elements fill their 6sshells first, then their 4f, 5d, and 6p shells, in that order; however, there are exceptions, such asgold.
This period contains thelanthanides, also known as therare earths. Many lanthanides are known for their magnetic properties, such asneodymium. Many period 6transition metals are very valuable, such asgold, however many period 6other metals are incredibly toxic, such asthallium. Period 6 contains the last stable element,lead. All subsequent elements in the periodic table areradioactive. Afterbismuth, which has a half-life or more than 1019 years,polonium,astatine, andradon are some of theshortest-lived and rarest elements known; less than a gram of astatine is estimated to exist on earth at any given time.[1]
| Chemical element | Block | Electron configuration | ||
|---|---|---|---|---|
| 55 | Cs | Caesium | s-block | [Xe] 6s1 |
| 56 | Ba | Barium | s-block | [Xe] 6s2 |
| 57 | La | Lanthanum | f-block[a] | [Xe] 5d1 6s2[b] |
| 58 | Ce | Cerium | f-block | [Xe] 4f1 5d1 6s2[b] |
| 59 | Pr | Praseodymium | f-block | [Xe] 4f3 6s2 |
| 60 | Nd | Neodymium | f-block | [Xe] 4f4 6s2 |
| 61 | Pm | Promethium | f-block | [Xe] 4f5 6s2 |
| 62 | Sm | Samarium | f-block | [Xe] 4f6 6s2 |
| 63 | Eu | Europium | f-block | [Xe] 4f7 6s2 |
| 64 | Gd | Gadolinium | f-block | [Xe] 4f7 5d1 6s2[b] |
| 65 | Tb | Terbium | f-block | [Xe] 4f9 6s2 |
| 66 | Dy | Dysprosium | f-block | [Xe] 4f10 6s2 |
| 67 | Ho | Holmium | f-block | [Xe] 4f11 6s2 |
| 68 | Er | Erbium | f-block | [Xe] 4f12 6s2 |
| 69 | Tm | Thulium | f-block | [Xe] 4f13 6s2 |
| 70 | Yb | Ytterbium | f-block | [Xe] 4f14 6s2 |
| 71 | Lu | Lutetium | d-block[a] | [Xe] 4f14 5d1 6s2 |
| 72 | Hf | Hafnium | d-block | [Xe] 4f14 5d2 6s2 |
| 73 | Ta | Tantalum | d-block | [Xe] 4f14 5d3 6s2 |
| 74 | W | Tungsten | d-block | [Xe] 4f14 5d4 6s2 |
| 75 | Re | Rhenium | d-block | [Xe] 4f14 5d5 6s2 |
| 76 | Os | Osmium | d-block | [Xe] 4f14 5d6 6s2 |
| 77 | Ir | Iridium | d-block | [Xe] 4f14 5d7 6s2 |
| 78 | Pt | Platinum | d-block | [Xe] 4f14 5d9 6s1[b] |
| 79 | Au | Gold | d-block | [Xe] 4f14 5d10 6s1[b] |
| 80 | Hg | Mercury | d-block | [Xe] 4f14 5d10 6s2 |
| 81 | Tl | Thallium | p-block | [Xe] 4f14 5d10 6s2 6p1 |
| 82 | Pb | Lead | p-block | [Xe] 4f14 5d10 6s2 6p2 |
| 83 | Bi | Bismuth | p-block | [Xe] 4f14 5d10 6s2 6p3 |
| 84 | Po | Polonium | p-block | [Xe] 4f14 5d10 6s2 6p4 |
| 85 | At | Astatine | p-block | [Xe] 4f14 5d10 6s2 6p5 |
| 86 | Rn | Radon | p-block | [Xe] 4f14 5d10 6s2 6p6 |
Caesium orcesium[note 1] is thechemical element with the symbolCs andatomic number 55. It is a soft, silvery-goldalkali metal with a melting point of 28 °C (82 °F), which makes it one of only five elemental metals that are liquid at (or near)room temperature.[note 2] Caesium is analkali metal and has physical and chemical properties similar to those ofrubidium andpotassium. The metal is extremely reactive andpyrophoric, reacting with water even at−116 °C (−177 °F). It is the leastelectronegative element having a stable isotope, caesium-133. Caesium is mined mostly frompollucite, while theradioisotopes, especiallycaesium-137, afission product, are extracted from waste produced bynuclear reactors.
Two German chemists,Robert Bunsen andGustav Kirchhoff, discovered caesium in 1860 by the newly developed method offlame spectroscopy. The first small-scale applications for caesium have been as a "getter" invacuum tubes and inphotoelectric cells. In 1967, a specific frequency from theemission spectrum of caesium-133 was chosen to be used in the definition of thesecond by theInternational System of Units. Since then, caesium has been widely used inatomic clocks.
Since the 1990s, the largestapplication of the element has been as caesium formate fordrilling fluids. It has a range of applications in the production of electricity, in electronics, and in chemistry. The radioactive isotope caesium-137 has ahalf-life of about 30 years and is used in medical applications, industrial gauges, and hydrology. Although the element is only mildly toxic, it is a hazardous material as a metal and its radioisotopes present a high health risk in case of radioactivity releases.
Barium is achemical element with the symbolBa andatomic number 56. It is the fifth element in Group 2, a soft silverymetallicalkaline earth metal. Barium is never found in nature in its pure form due to itsreactivity withair. Its oxide is historically known asbaryta but it reacts with water and carbon dioxide and is not found as a mineral. The most common naturally occurring minerals are the very insoluble barium sulfate, BaSO4 (barite), andbarium carbonate, BaCO3(witherite). Barium's name originates fromGreekbarys (βαρύς), meaning "heavy", describing the high density of some common barium-containing ores.
Barium has few industrial applications, but the metal has been historically used toscavenge air invacuum tubes. Barium compounds impart a green color to flames and have been used in fireworks.Barium sulfate is used for its density, insolubility, and X-ray opacity. It is used as an insoluble heavy additive to oil well drilling mud, and in purer form, as an X-rayradiocontrast agent for imaging the human gastrointestinal tract. Soluble barium compounds are poisonous due to release of the soluble barium ion, and have been used as rodenticides. New uses for barium continue to be sought. It is a component of some "high temperature"YBCOsuperconductors, and electroceramics.
Thelanthanide orlanthanoid (IUPAC nomenclature)[10] series comprises the fifteenmetallicchemical elements withatomic numbers 57 through 71, fromlanthanum throughlutetium.[1]: 240 [11][12] These fifteen elements, along with the chemically similar elementsscandium andyttrium, are often collectively known as therare-earth elements.
The informal chemical symbolLn is used in general discussions of lanthanide chemistry. All but one of the lanthanides aref-block elements, corresponding to the filling of the 4felectron shell;lanthanum, ad-block element, is also generally considered to be a lanthanide due to its chemical similarities with the other fourteen. All lanthanide elements form trivalent cations, Ln3+, whose chemistry is largely determined by theionic radius, which decreases steadily from lanthanum to lutetium.
| Chemical element | La | Ce | Pr | Nd | Pm | Sm | Eu | Gd | Tb | Dy | Ho | Er | Tm | Yb | Lu |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Atomic number | 57 | 58 | 59 | 60 | 61 | 62 | 63 | 64 | 65 | 66 | 67 | 68 | 69 | 70 | 71 |
| Image |  |  |  |  |  |  |  |  |  |  |  |  |  |  | |
| Density (g/cm3) | 6.162 | 6.770 | 6.77 | 7.01 | 7.26 | 7.52 | 5.244 | 7.90 | 8.23 | 8.540 | 8.79 | 9.066 | 9.32 | 6.90 | 9.841 |
| Melting point (°C) | 920 | 795 | 935 | 1024 | 1042 | 1072 | 826 | 1312 | 1356 | 1407 | 1461 | 1529 | 1545 | 824 | 1652 |
| Atomicelectron configuration* | 5d1 | 4f15d1 | 4f3 | 4f4 | 4f5 | 4f6 | 4f7 | 4f75d1 | 4f9 | 4f10 | 4f11 | 4f12 | 4f13 | 4f14 | 4f145d1 |
| Ln3+ electron configuration*[13] | 4f0[14] | 4f1 | 4f2 | 4f3 | 4f4 | 4f5 | 4f6 | 4f7 | 4f8 | 4f9 | 4f10 | 4f11 | 4f12 | 4f13 | 4f14 |
| Ln3+ radius (pm)[15] | 103 | 102 | 99 | 98.3 | 97 | 95.8 | 94.7 | 93.8 | 92.3 | 91.2 | 90.1 | 89 | 88 | 86.8 | 86.1 |
The lanthanide elements are the group of elements withatomic number increasing from 57 (lanthanum) to 71 (lutetium). They are termed lanthanide because the lighter elements in the series are chemically similar tolanthanum. Strictly speaking, both lanthanum and lutetium have been labeled asgroup 3 elements, because they both have a single valence electron in the d shell. However, both elements are often included in any general discussion of the chemistry of the lanthanide elements.
In presentations of theperiodic table, thelanthanides and theactinides are customarily shown as two additional rows below the main body of the table,[1] with placeholders or else a selected single element of each series (eitherlanthanum orlutetium, and eitheractinium orlawrencium, respectively) shown in a single cell of the main table, betweenbarium andhafnium, andradium andrutherfordium, respectively. This convention is entirely a matter ofaesthetics and formatting practicality; a rarely usedwide-formatted periodic table inserts the lanthanide and actinide series in their proper places, as parts of the table's sixth and seventh rows (periods).
Lutetium (/ljuːˈtiːʃiəm/lew-TEE-shee-əm) is achemical element with the symbolLu andatomic number 71. It is the last element in thelanthanide series, which, along with thelanthanide contraction, explains several important properties of lutetium, such as it having the highest hardness or density among lanthanides. Unlike other lanthanides, which lie in thef-block of theperiodic table, this element lies in thed-block; however,lanthanum is sometimes placed on the d-block lanthanide position. Chemically, lutetium is a typical lanthanide: its only common oxidation state is +3, seen in its oxide, halides and other compounds. In an aqueous solution, like compounds of other late lanthanides, soluble lutetium compounds form a complex with nine water molecules.
Lutetium was independently discovered in 1907 by French scientistGeorges Urbain, Austrian mineralogist BaronCarl Auer von Welsbach, and American chemistCharles James. All of these men found lutetium as an impurity in the mineralytterbia, which was previously thought to consist entirely of ytterbium. The dispute on the priority of the discovery occurred shortly after, with Urbain and von Welsbach accusing each other of publishing results influenced by the published research of the other; the naming honor went to Urbain as he published his results earlier. He chose the name lutecium for the new element but in 1949 the spelling of element 71 was changed to lutetium. In 1909, the priority was finally granted to Urbain and his names were adopted as official ones; however, the name cassiopeium (or later cassiopium) for element 71 proposed by von Welsbach was used by many German scientists until the 1950s. Like other lanthanides, lutetium is one of the elements that traditionally were included in the classification "rare earths."
Lutetium is rare and expensive; consequently, it has few specific uses. For example, aradioactive isotope lutetium-176 is used innuclear technology to determine the age ofmeteorites. Lutetium usually occurs in association with the elementyttrium and is sometimes used in metalalloys and as acatalyst in various chemical reactions.177Lu-DOTA-TATE is used for radionuclide therapy (seeNuclear medicine) on neuroendocrine tumours.
Hafnium is achemical element with thesymbolHf andatomic number 72. Alustrous, silvery gray,tetravalenttransition metal, hafnium chemically resembleszirconium and is found in zirconiumminerals. Its existence waspredicted by Dmitri Mendeleev in 1869. Hafnium was the penultimatestable isotope element to be discovered (rhenium was identified two years later). Hafnium is named forHafnia, theLatin name for "Copenhagen", where it was discovered.
Hafnium is used in filaments and electrodes. Somesemiconductor fabrication processes use its oxide forintegrated circuits at 45 nm and smaller feature lengths. Somesuperalloys used for special applications contain hafnium in combination withniobium,titanium, ortungsten.
Hafnium's largeneutron capture cross-section makes it a good material forneutron absorption incontrol rods innuclear power plants, but at the same time requires that it be removed from the neutron-transparent corrosion-resistant zirconium alloys used in nuclear reactors.
Tantalum is achemical element with the symbolTa andatomic number 73. Previously known astantalium, the name comes fromTantalus, a character from Greek mythology.[16] Tantalum is a rare, hard, blue-gray,lustroustransition metal that is highly corrosion resistant. It is part of therefractory metals group, which are widely used as minor component in alloys. The chemical inertness of tantalum makes it a valuable substance for laboratory equipment and a substitute forplatinum, but its main use today is intantalum capacitors inelectronic equipment such asmobile phones,DVD players,video game systems andcomputers.Tantalum, always together with the chemically similarniobium, occurs in themineralstantalite,columbite andcoltan (a mix of columbite and tantalite).
Tungsten, also known aswolfram, is achemical element with the chemical symbolW andatomic number 74. The wordtungsten comes from the Swedish languagetung sten directly translatable toheavy stone,[17] though the name isvolfram in Swedish to distinguish it fromScheelite, in Swedish alternatively namedtungsten.
A hard, raremetal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as a metal in 1783. Its importantores includewolframite andscheelite. Thefree element is remarkable for its robustness, especially the fact that it has the highestmelting point of all the non-alloyed metals and the second highest of all the elements aftercarbon. Also remarkable is its high density of 19.3 times that of water, comparable to that ofuranium andgold, and much higher (about 1.7 times) than that oflead.[18] Tungsten with minor amounts of impurities is oftenbrittle[19] andhard, making it difficult towork. However, very pure tungsten, though still hard, is moreductile, and can be cut with a hard-steelhacksaw.[20]
The unalloyed elemental form is used mainly in electrical applications. Tungsten's many alloys have numerous applications, most notably in incandescentlight bulb filaments,X-ray tubes (as both the filament and target), electrodes inTIG welding, andsuperalloys. Tungsten's hardness and highdensity give it military applications in penetratingprojectiles. Tungsten compounds are most often used industrially ascatalysts.
Tungsten is the only metal from the thirdtransition series that is known to occur inbiomolecules, where it is used in a few species of bacteria. It is the heaviest element known to be used by any living organism. Tungsten interferes withmolybdenum andcopper metabolism, and is somewhat toxic to animal life.[21][22]
Rhenium is achemical element with the symbolRe andatomic number 75. It is a silvery-white, heavy, third-rowtransition metal ingroup 7 of theperiodic table. With anestimated average concentration of 1part per billion (ppb), rhenium is one of the rarest elements in theEarth's crust. The free element has thethird-highestmelting point and highest boiling point of any element. Rhenium resemblesmanganese chemically and is obtained as aby-product ofmolybdenum andcopper ore's extraction and refinement. Rhenium shows in its compounds a wide variety ofoxidation states ranging from −1 to +7.
Discovered in 1925, rhenium was the laststable element to be discovered. It was named after the riverRhine in Europe.
Nickel-basedsuperalloys of rhenium are used in the combustion chambers, turbine blades, and exhaust nozzles ofjet engines, these alloys contain up to 6% rhenium, making jet engine construction the largest single use for the element, with the chemical industry's catalytic uses being next-most important. Because of the low availability relative to demand, rhenium is among the most expensive of metals, with an average price of approximately US$4,575 perkilogram (US$142.30 per troy ounce) as of August 2011; it is also of critical strategic military importance, for its use in high performance military jet and rocket engines.[23]
Osmium is achemical element with the symbolOs andatomic number 76. It is a hard, brittle, blue-gray or blue-blacktransition metal in theplatinum family and is the densest naturally occurring element, with adensity of22.59 g/cm3 (slightly greater than that ofiridium and twice that oflead). It is found in nature as an alloy, mostly in platinum ores; itsalloys withplatinum,iridium, and other platinum group metals are employed infountain pen tips, electrical contacts, and other applications where extreme durability and hardness are needed.[24]
Iridium is thechemical element withatomic number 77, and is represented by the symbolIr. A very hard, brittle, silvery-whitetransition metal of theplatinum family, iridium is the second-densest element (afterosmium) and is the mostcorrosion-resistant metal, even at temperatures as high as 2000 °C. Although only certain molten salts andhalogens are corrosive to solid iridium, finely divided iridium dust is much more reactive and can be flammable.
Iridium was discovered in 1803 among insoluble impurities in naturalplatinum.Smithson Tennant, the primary discoverer, named the iridium for the goddessIris, personification of the rainbow, because of the striking and diverse colors of its salts. Iridium isone of the rarest elements in theEarth's crust, with annual production and consumption of only threetonnes.191
Ir and193
Ir are the only two naturally occurringisotopes of iridium as well as the onlystable isotopes; the latter is the more abundant of the two.
The most important iridium compounds in use are the salts and acids it forms withchlorine, though iridium also forms a number oforganometallic compounds used in industrialcatalysis, and in research. Iridium metal is employed when high corrosion resistance at high temperatures is needed, as in high-endspark plugs,crucibles for recrystallization of semiconductors at high temperatures, and electrodes for the production of chlorine in thechloralkali process. Iridium radioisotopes are used in someradioisotope thermoelectric generators.
Iridium is found in meteorites with an abundance much higher than its average abundance in the Earth's crust. For this reason the unusually high abundance of iridium in the clay layer at theCretaceous–Paleogene boundary gave rise to theAlvarez hypothesis that the impact of a massive extraterrestrial object caused the extinction of dinosaurs and many other species 66 million years ago. It is thought that the total amount of iridium in the planet Earth is much higher than that observed in crustal rocks, but as with other platinum group metals, the high density andtendency of iridium to bond with iron caused most iridium to descend below the crust when the planet was young and still molten.
Platinum is achemical element with thechemical symbolPt and anatomic number of 78.
Its name is derived from the Spanish termplatina, which is literally translated into "little silver".[25][26] It is adense,malleable,ductile,precious, gray-whitetransition metal.
Platinum has six naturally occurringisotopes. It is one of therarest elements in the Earth's crust and has an average abundance of approximately 5 μg/kg. It is theleast reactive metal. It occurs in somenickel andcopper ores along with some native deposits, mostly in South Africa, which accounts for 80% of the world production.
As a member of theplatinum group of elements, as well as of thegroup 10 of theperiodic table of elements, platinum is generally non-reactive. It exhibits a remarkable resistance to corrosion, even at high temperatures, and as such is considered anoble metal. As a result, platinum is often found chemically uncombined as native platinum. Because it occurs naturally in thealluvial sands of various rivers, it was first used bypre-Columbian South American natives to produce artifacts. It was referenced in European writings as early as 16th century, but it was not untilAntonio de Ulloa published a report on a new metal ofColombian origin in 1748 that it became investigated by scientists.
Platinum is used incatalytic converters, laboratory equipment,electrical contacts andelectrodes,platinum-resistance thermometers,dentistry equipment, and jewelry. Because only a few hundred tonnes are produced annually, it is a scarce material, and is highly valuable. Being aheavy metal, it leads to health issues upon exposure to its salts, but due to its corrosion resistance, it is not as toxic as some metals.[27] Its compounds, most notablycisplatin, are applied inchemotherapy against certain types of cancer.[28]
Gold is a dense, soft, shiny, malleable and ductile metal. It is achemical element with the symbolAu andatomic number 79.
Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is atransition metal and agroup 11 element. It is one of the least reactive chemical elements solid under standard conditions. The metal therefore occurs often in free elemental (native) form, asnuggets or grains in rocks, inveins and inalluvial deposits. Less commonly, it occurs in minerals as gold compounds, usually withtellurium.
Gold resists attacks by individual acids, but it can be dissolved by theaqua regia (nitro-hydrochloric acid), so named because it dissolves gold. Gold also dissolves in alkaline solutions ofcyanide, which have been used in mining. Gold dissolves inmercury, formingamalgam alloys. Gold is insoluble innitric acid, which dissolvessilver andbase metals, a property that has long been used to confirm the presence of gold in items, giving rise to the termtheacid test.
Gold has been a valuable and highly sought-afterprecious metal forcoinage, jewelry, and other arts since long before the beginning ofrecorded history.Gold standards have been a common basis formonetary policies throughout human history,[citation needed] later being supplanted byfiat currency starting in the 1930s. The lastgold certificate andgold coin currencies were issued in the U.S. in 1932. In Europe, most countries left the gold standard with the start ofWorld War I in 1914 and, with huge war debts, failed to return to gold as a medium of exchange.
A total of 165,000tonnes of gold have been mined in human history, as of 2009.[29] This is roughly equivalent to 5.3 billiontroy ounces or, in terms of volume, about 8500 m3, or acube 20.4 m on a side. The world consumption of new gold produced is about 50% in jewelry, 40% in investments, and 10% in industry.[30]
Besides its widespread monetary and symbolic functions, gold has many practical uses indentistry,electronics, and other fields. Its highmalleability,ductility, resistance to corrosion and most other chemical reactions, and conductivity of electricity led to many uses of gold, includingelectric wiring, colored-glass production and evengold leaf eating.
It has been claimed that most of the Earth's gold lies at its core, the metal's high density having made it sink there in the planet's youth. Virtually all of the gold that mankind has discovered is considered to have been deposited later bymeteorites which contained the element. This supposedly explains why, in prehistory, gold appeared as nuggets on the earth's surface.[31][32][33][34][35]
Mercury is achemical element with the symbolHg andatomic number 80. It is also known asquicksilver orhydrargyrum ( < Greek "hydr-"water and "argyros"silver). A heavy, silveryd-block element, mercury is the only metal that is liquid atstandard conditions for temperature and pressure; the only other element that is liquid under these conditions isbromine, though metals such ascaesium,francium,gallium, andrubidium melt just above room temperature. With afreezing point of −38.83 °C andboiling point of 356.73 °C, mercury has one of the narrowest ranges of its liquid state of any metal.[36][37][38]
Mercury occurs in deposits throughout the world mostly ascinnabar (mercuric sulfide). The red pigmentvermilion is mostly obtained by reduction from cinnabar. Cinnabar is highly toxic by ingestion or inhalation of the dust.Mercury poisoning can also result from exposure to water-soluble forms of mercury (such asmercuric chloride ormethylmercury), inhalation of mercury vapor, or eating seafood contaminated with mercury.
Mercury is used inthermometers,barometers,manometers,sphygmomanometers,float valves,mercury switches, and other devices though concerns about the element's toxicity have led to mercury thermometers and sphygmomanometers being largely phased out in clinical environments in favor ofalcohol-filled,galinstan-filled, digital, orthermistor-based instruments. It remains in use in scientific research applications and inamalgam material fordental restoration. It is used in lighting: electricity passed through mercury vapor in a phosphor tube produces short-waveultraviolet light which then causes the phosphor tofluoresce, making visible light.
Thallium is a chemical element with the symbolTl and atomic number 81. This soft grayother metal resemblestin but discolors when exposed to air. The two chemistsWilliam Crookes andClaude-Auguste Lamy discovered thallium independently in 1861 by the newly developed method offlame spectroscopy. Both discovered the new element in residues ofsulfuric acid production.
Approximately 60–70% of thallium production is used in theelectronics industry, and the remainder is used in thepharmaceutical industry and inglass manufacturing.[39] It is also used ininfrared detectors. Thallium is highlytoxic and was used inrat poisons andinsecticides. Its use has been reduced or eliminated in many countries because of its nonselective toxicity. Because of its use formurder, thallium has gained the nicknames "The Poisoner's Poison" and "Inheritance Powder" (alongsidearsenic).[40]
Lead is a main-groupelement in thecarbon group with the symbolPb (fromLatin:plumbum) andatomic number 82. Lead is a soft,malleableother metal. It is also counted as one of theheavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed to air. Lead has a shiny chrome-silver luster when it is melted into a liquid.
Lead is used in building construction,lead-acid batteries,bullets andshots, weights, as part ofsolders,pewters,fusible alloys and as aradiation shield. Lead has the highestatomic number of all of thestable elements, although the next higher element,bismuth, has ahalf-life that is so long (much longer than the age of the universe) that it can be considered stable. Its four stable isotopes have 82protons, amagic number in thenuclear shell model ofatomic nuclei.
Lead, at certain exposure levels, is a poisonous substance to animals as well as for human beings. It damages thenervous system and causesbrain disorders. Excessive lead also causes blood disorders in mammals. Like the elementmercury, another heavy metal, lead is aneurotoxin that accumulates both in soft tissues and the bones.Lead poisoning has been documented fromancient Rome,ancient Greece, andancient China.
Bismuth is achemical element with symbolBi andatomic number 83. Bismuth, a trivalentother metal, chemically resemblesarsenic andantimony. Elemental bismuth may occur naturally uncombined, although its sulfide and oxide form important commercial ores. Thefree element is 86% as dense aslead. It is a brittle metal with a silvery white color when newly made, but often seen in air with a pink tinge owing to the surface oxide. Bismuth metal has been known from ancient times, although until the 18th century it was often confused with lead and tin, which each have some of bismuth's bulk physical properties. The etymology is uncertain but possibly comes from Arabicbi ismid meaning having the properties of antimony[41] or German wordsweisse masse orwismuth meaning "white mass".[42]
Bismuth is the most naturallydiamagnetic of all metals, and onlymercury has a lowerthermal conductivity.
Bismuth has classically been considered to be the heaviest naturally occurring stable element, in terms of atomic mass. Recently, however, it has been found to be very slightly radioactive: its only primordial isotopebismuth-209 decays viaalpha decay intothallium-205 with ahalf-life of more than abillion times the estimatedage of the universe.[43]
Bismuth compounds (accounting for about half the production of bismuth) are used incosmetics, pigments, and a few pharmaceuticals. Bismuth has unusually lowtoxicity for a heavy metal. As the toxicity oflead has become more apparent in recent years, alloy uses for bismuth metal (presently about a third of bismuth production), as a replacement for lead, have become an increasing part of bismuth's commercial importance.
Polonium is achemical element with the symbolPo andatomic number 84, discovered in 1898 byMarie Skłodowska-Curie andPierre Curie. A rare and highlyradioactive element, polonium is chemically similar tobismuth[44] andtellurium, and it occurs inuraniumores. Polonium has been studied for possible use in heatingspacecraft. As it is unstable, allisotopes of polonium are radioactive. There is disagreement as to whether polonium is apost-transition metal ormetalloid.[45][46]
Astatine is aradioactivechemical element with the symbolAt andatomic number 85. It occurs on the Earth only as the result of decay of heavier elements, and decays away rapidly, so much less is known about this element than its upper neighbors in theperiodic table. Earlier studies have shown this element follows periodic trends, being the heaviest knownhalogen, withmelting andboiling points being higher than those of lighter halogens.
Until recently most of the chemical characteristics of astatine were inferred from comparison with other elements; however, important studies have already been done. The main difference between astatine andiodine is that the HAt molecule is chemically ahydride rather than ahalide; however, in a fashion similar to the lighter halogens, it is known to form ionic astatides with metals. Bonds tononmetals result in positiveoxidation states, with +1 best portrayed by monohalides and their derivatives, while the higher are characterized by bond to oxygen and carbon. Attempts to synthesize astatine fluoride have been met with failure. The second longest-living astatine-211 is the only one to find a commercial use, being useful as analpha emitter in medicine; however, only extremely small quantities are used, and in larger ones it is very hazardous, as it is intensely radioactive.
Astatine was first produced byDale R. Corson,Kenneth Ross MacKenzie, andEmilio Segrè in theUniversity of California, Berkeley in 1940. Three years later, it was found in nature; however, with an estimated amount of less than 28 grams (1 oz) at given time, astatine is the least abundant element in Earth's crust among non-transuranium elements. Among astatine isotopes, four (withmass numbers 215, 217, 218 and 219) are present in nature as the result of decay of heavier elements; however, the most stable astatine-210 and the industrially used astatine-211 are not.
Radon is achemical element with symbolRn andatomic number 86. It is aradioactive, colorless, odorless, tasteless[47]noble gas, occurring naturally as the decay product ofuranium orthorium. Its most stableisotope,222Rn, has ahalf-life of 3.8 days. Radon is one of the densest substances that remains agas under normal conditions. It is also the only gas that is radioactive under normal conditions, and is considered a health hazard due to its radioactivity. Intense radioactivity also hindered chemical studies of radon and only a few compounds are known.
Radon is formed as part of the normal radioactivedecay chain of uranium and thorium. Uranium and thorium have been around since the earth was formed and theirmost common isotope has a very long half-life (14.05 billion years). Uranium and thorium,radium, and thus radon, will continue to occur for millions of years at about the same concentrations as they do now.[48] As the radioactive gas of radon decays, it produces new radioactive elements called radon daughters or decay products. Radon daughters are solids and stick to surfaces such as dust particles in the air. If contaminated dust is inhaled, these particles can stick to the airways of the lung and increase the risk of developing lung cancer.[49]
Radon is responsible for the majority of the public exposure toionizing radiation. It is often the single largest contributor to an individual'sbackground radiation dose, and is the most variable from location to location. Radon gas from natural sources can accumulate in buildings, especially in confined areas such as attics and basements. It can also be found in somespring waters and hot springs.[50]
Epidemiological studies have shown a clear link between breathing high concentrations of radon and incidence oflung cancer. Thus, radon is considered a significant contaminant that affectsindoor air quality worldwide. According to theUnited States Environmental Protection Agency, radon is the second most frequent cause of lung cancer, after cigarette smoking, causing 21,000 lung cancer deaths per year in theUnited States. About 2,900 of these deaths occur among people who have never smoked. While radon is the second most frequent cause of lung cancer, it is the number one cause among non-smokers, according to EPA estimates.[51]
Of the period 6 elements, only tungsten and the early lanthanides[52] are known to have any biological role in organisms, and even then only in lower organisms (not mammals). However, gold, platinum, mercury, and some lanthanides such as gadolinium have applications as drugs.
Most of the period 6 elements are toxic (for instance lead) and produceheavy-element poisoning. Promethium, polonium, astatine and radon are radioactive, and therefore present radioactive hazards.
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