Chalcogen

Template:Periodic table (chalcogens) The chalcogens (/ˈkælkədʒn/) are the chemical elements in group 16 (old styles: VIB and VIA) of the periodic table. This group is also known as the oxygen family. It consists of the elements oxygen (O), sulfur (S), selenium (Se), tellurium (Te), the radioactive element polonium (Po), and the synthetic element livermorium (Lv).

The chalcogens are all two electrons short of a full outer shell. The most common oxidation states for the chalcogens are ± 2, 4, and 6. The chalcogens have relatively low atomic radii, especially the lighter chalcogens.

Oxygen is generally extracted from air. Sulfur is extracted from oil and natural gas. Selenium and tellurium are byproducts of copper refining. Polonium is usually made in nuclear reactors, and livermorium is always made in nuclear reactors.

The lighter chalcogens are typically nontoxic in their elemental form, and are in fact often critical to life, while the heavier chalcogens are typically toxic.

The word chalcogen comes from the Greek word chalkos, meaning "bronze" or "ore" and the genēs, meaning "born". These electronegative elements are strongly associated with metal-bearing minerals, where they have formed water-insoluble compounds with the metals in the ores.

Properties

Atomic and physical

Members of this group show similar patterns in their electron configuration, especially the outermost shells, resulting in similar trends in chemical behavior:

Z	Element	No. of electrons/shell
8	oxygen	2, 6
16	sulfur	2, 8, 6
34	selenium	2, 8, 18, 6
52	tellurium	2, 8, 18, 18, 6
84	polonium	2, 8, 18, 32, 18, 6
116	livermorium	2, 8, 18, 32, 32, 18, 6

Oxygen, sulfur, and selenium are nonmetals, and tellurium is a metalloid semiconductor, meaning that its electrical properties are between those of a metal and an insulator. Not much is known about polonium's properties, but it is thought to be a metal.

All chalcogens have six valence electrons. Most of the solid chalcogens are soft and do not conduct heat well. Electronegativity decreases towards the chalcogens with higher atomic numbers. Density, and melting and boiling points tend to increase towards the chalcogens with higher atomic numbers. The chalcogens have varying crystal structures. Oxygen's crystal structure is monoclinic. Sulfur's is orthorhombic. Selenium and tellurium have the hexagonal crystal structure. Polonium's crystal structure is cubic.

Isotopes

Out of the six known chalcogens, three (oxygen, tellurium, and polonium) have atomic numbers equal to or near a nuclear magic number. Oxygen has three stable isotopes (Oxygen-16, Oxygen-17, Oxygen-18) and 14 radioactive isotopes. Sulfur has four stable isotopes (Sulfur-32, Sulfur-33, Sulfur-34, and Sulfur-36) and 20 radioactive ones. Selenium has 6 observationally stable or nearly stable isotopes (Selenium-74, Selenium-76, Selenium-77, Selenium-78, Selenium-80, and Selenium-82) and 26 radioactive isotopes. Tellurium has 8 stable or nearly stable isotopes (Tellurium-120, Tellurium-122, Tellurium-123, Tellurium-124, Tellurium-125, Tellurium-126, Tellurium-128, Tellurium-130, and Tellurium-132) and 31 unstable ones. Polonium has 42 isotopes, none of which are stable. However, in addition to the stable isotopes, some radioactive chalcogen isotopes occur in nature, either because they are decay products, such as polonium-210, or because they are primordial, such as selenium-82.

Chemical

From a chemical perspective, oxygen, sulfur, and selenium are classified as nonmetals, tellurium as a metalloid, and polonium as a metal. The formal oxidation number of the most common chalcogen compounds is −2. Other values, such as −1 in pyrite and peroxide, can be attained. The highest formal oxidation number +6 is found in sulfates, selenates and tellurates, such as in sulfuric acid or sodium selenate (Na₂SeO₄).

There are multiple acids containing chalcogens; some of these are H₂SO₄, H₂SO₃, H₂SeO₄, and H₂TeO₄. Almost all chalcogen hydrides are toxic; the sole exception is water. Oxygen ions often come in the forms of peroxide ions (O₂^2-), and hydroxide ions (OH^1-). Sulfur ions generally come in the form of sulfides (S^2-), thiosulfates (S₂O₃^2-). Selenium ions usually come in the form of selenides (Se^2-), and selenates (SeO₄^2-). Tellurium ions often come in the form of tellurates (TeO₄^2-). Molecules containing metal bonded to chalcogens are common as minerals. For example, pyrite (FeS₂) is an iron ore. The rare mineral calaverite is the ditelluride AuTe₂.

Water (H
₂O) is the most familiar chalcogen-containing compound.

Oxygen is the second most electronegative element (asides from fluorine) and forms compounds with almost all of the chemical elements, including some fo the noble gases. It commonly bonds with many metals and metalloids to form oxides, including iron oxide, titanium oxide, and silicon oxide. Oxygen's most common oxidation state is -2, and the oxidation state -1 is also common. With hydrogen it forms water and hydrogen peroxide. Organic oxygen compounds are ubiquitous in organic chemistry.

Sulfur's oxidation states are -6, -4, -2, +2, +4, and +6. Sulfur-containing analogs of oxygen compounds often have the prefix thio-. Sulfur's chemistry is very similar to oxygen's, except for several respects. Among these is the fact that while sulfur double bonds are far weaker than oxygen double bonds, sulfur single bonds are weaker than oxygen single bonds. Organic sulfur compounds are utilized by some organisms.

Selenium's oxidation states are -2, +4, and +6. Selenium, like most chalcogens, bonds with oxygen. There are some organic selenium compounds, such as selenoproteins. Tellurium's oxidation states are -2, +2, +4, and +6. Tellurium forms the oxides TeO, TeO₂, and TeO₃. Polonium's oxidation states are +2 and +4.

Although all group 16 elements of the periodic table, including oxygen, are defined as chalcogens, oxygen and oxides are usually distinguished from chalcogens and chalcogenides. The term chalcogenide is more commonly reserved for sulfides,selenides, and tellurides, rather than for oxides. Binary compounds of the chalcogens are called chalcogenides (rather than chalcides; however, this breaks the pattern of halogen/halide and pnictogen/pnictide).

Chalcophile elements

Chalcophile elements are those that remain on or close to the surface because they combine readily with sulfur and/or some other chalcogen other than oxygen, forming compounds which do not sink into the core. In the Goldschmidt classification of elements, the chalcophile elements include the chalcogens themselves (except for oxygen), as they combine with each other, as well as Ag, As, Bi,Cd, Cu, Ga, Ge,Hg, In, Pb, Po,S, Sb, Se, Sn,Te, Tl, and Zn. Chalcophile ("chalcogen-loving") elements in this context are those metals (sometimes called "poor metals") and heavier nonmetals that have a low affinity for oxygen and prefer to bond with the heavier chalcogen sulfur as sulfides. Because sulfides are much denser than the silicate minerals formed by lithophile elements, chalcophile elements have separated below the lithophiles at the time of the first crystallisation of the Earth's crust. This has led to their depletion in the Earth's crust relative to their solar abundances, though because the minerals they form are nonmetallic, this depletion has not reached the levels found with siderophile elements.

Goldschmidt classification in the periodic table

	1	2		3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18
Group →
↓ Period
1	1 H																		2 He
2	3 Li	4 Be												5 B	6 C	7 N	8 O	9 F	10 Ne
3	11 Na	12 Mg												13 Al	14 Si	15 P	16 S	17 Cl	18 Ar
4	19 K	20 Ca		21 Sc	22 Ti	23 V	24 Cr	25 Mn	26 Fe	27 Co	28 Ni	29 Cu	30 Zn	31 Ga	32 Ge	33 As	34 Se	35 Br	36 Kr
5	37 Rb	38 Sr		39 Y	40 Zr	41 Nb	42 Mo	43 Tc	44 Ru	45 Rh	46 Pd	47 Ag	48 Cd	49 In	50 Sn	51 Sb	52 Te	53 I	54 Xe
6	55 Cs	56 Ba		71 Lu	72 Hf	73 Ta	74 W	75 Re	76 Os	77 Ir	78 Pt	79 Au	80 Hg	81 Tl	82 Pb	83 Bi	84 Po	85 At	86 Rn
7	87 Fr	88 Ra		103 Lr	104 Rf	105 Db	106 Sg	107 Bh	108 Hs	109 Mt	110 Ds	111 Rg	112 Cn	113 Nh	114 Fl	115 Mc	116 Lv	117 Ts	118 Og
				57 La	58 Ce	59 Pr	60 Nd	61 Pm	62 Sm	63 Eu	64 Gd	65 Tb	66 Dy	67 Ho	68 Er	69 Tm	70 Yb
				89 Ac	90 Th	91 Pa	92 U	93 Np	94 Pu	95 Am	96 Cm	97 Bk	98 Cf	99 Es	100 Fm	101 Md	102 No

Goldschmidt classification: Lithophile Siderophile Chalcophile Atmophile Trace/Synthetic

Allotropes

See also: Allotropes of oxygen and Allotropes of sulfur

Phase diagram of sulfur showing the relative stabilities of several allotropes

Oxygen's most common allotrope is diatomic oxygen, or O_{2, which is two oxygen atoms bonded together linearly. Another well-known allotrope of oxygen is O3, or ozone, which is three oxygen atoms bonded together in a bent formation. There is also an allotrope called tetraoxygen, or O4, and six allotropes of solid oxygen. One allotrope of solid oxygen has the formula O8, and is called "red oxygen" because it is red as opposed to the blue color or colorlessness of the other known allotropes of oxygen.}

Sulfur has over 20 known allotropes, and in this respect is second only to carbon. The most common allotropes are in the form of eight-atom rings, but other molecular allotropes are known containing as little as two atoms or as many as 20. The most notable sulfur allotropes are rhombic sulfur and monoclinic sulfur. Rhombic sulfur is the more stable of the two allotropes. Monoclinic sulfur takes the form of long needles and is formed when liquid sulfur is cooled to slightly below its melting point. The atoms in liquid sulfur are generally in the form of long chains, but above 190° Celsius, the chains begin to break down. If liquid sulfur above 190° Celsius is frozen very rapidly, the resulting sulfur is amorphous or "plastic" sulfur. Gaseous sulfur is a mixture of diatomic sulfur (S₂) and 8-atom rings.

Selenium has at least five known allotropes. The gray allotrope, commonly referred to as the "metallic" allotrope, despite not being a metal, is stable and has a hexagonal crystal structure. The gray allotrope of selenium is soft, with a Mohs hardness of 2, and brittle. The four other allotropes of selenium are metastable. These include two monoclinic red allotropes and two amorphous allotropes, one of which is red and one of which is black.

Tellurium is not known to have any allotropes, while polonium has two allotropes: α-polonium and β-polonium.

History

Early discoveries

Sulfur was known in the ancient history and it is mentioned in Bible 15 times. Sulfur was known to the ancient Greeks commonly mined by the ancient Romans. In the Middle Ages, sulfur was a key part of alchemical experiments. Sulfur was also used in ancient times as a component of Greek fire. In the 1700s and 1800s, scientists Louis-Josef Gay-Lussac and Louis-Jacques Thénard proved sulfur to be a chemical element.

Early attempts to discover oxygen from air were hampered by the fact that air was thought of as a single element up to the 17th and 18th centuries. In the 1600s and 1700s, Robert Hooke, Mikhail Lomonosov, Ole Borch, and Pierre Bayden all successfully created oxygen, but did not realize it at the time. Oxygen was finally officially discovered by Joseph Priestly in 1774. Priestly created oxygen focusing sunlight on a sample of mercuric oxide and collected the resulting gas. However, Carl Wilhelm Scheele also created oxygen in 1771 by the same method as Priestly. However, Scheele did not publish his results until 1777.

Tellurium was first discovered in 1783 by Franz Joseph Müller von Reichenstein. He discovered tellurium in a sample of what is now known as calaverite. Müller assumed at first that the sample was pure antimony, but tests he ran on the sample did not agree with this. Muller then guessed that it was bismuth sulfide, but tests confirmed that the sample was not that. For some years, Muller pondered the problem. Eventually he realized that the sample was gold bonded with an unknown element. In 1796, Müller sent part of the sample to the German chemist Martin Klaproth, who purified the undiscovered element. Klaproth decided to call the element tellurium after the Latin word for earth.

Selenium was discovered in 1817 by Jöns Jacob Berzelius. Berzelius discovered a reddish-brown sediment at a sulfuric acid manufacturing plant. He initially thought that the sediment contained tellurium, but came to realize that it was in fact a new element, which he named selenium after the Greek word for moon.

Periodic table placing

Mendeleev's periodic system proposed in 1871 showing oxygen, sulfur, selenium and tellurium part of his group VI

Three of the chalcogens (sulfur, selenium, and tellurium) were part of the discovery of periodicity, as they are among a series of triads of elements in the same group that were noted by Johann Wolfgang Döbereiner as having similar properties. Around 1865 John Newlands produced a series of papers where he listed the elements in order of increasing atomic weight, similar physical and chemical properties recurred at intervals of eight; he likened such periodicity to the octaves of music. His version included a "group b" consisting of oxygen, sulfur, selenium, tellurium, and osmium.

After 1869, Dmitri Mendeleev proposed his periodic table placing oxygen at the top of "group VI" along sulfur, selenium, and tellurium . Chromium, molybdenum, tungsten, and uranium were sometimes included in this group, but they would be later rearranged as part of group VIB and actinides, respectively. Oxygen along sulfur, selenium tellurium and later polonium would be grouped in group VIA, until switching to group 16 in 1988.

Modern discoveries

In the late 19 th century, Marie Curie and Pierre Curie discovered that a sample of pitchblende was emitting four times as much radioactivity as could be explained by the presence of uranium alone. The Curies gathered several tons of pitchblende and refined it for several months until had a pure sample of polonium. The discovery officially took place in 1898. Prior to the invention of nuclear reactors, the only way to create polonium was to extract it over several months from uranium ore.

The first attempt at creating livermorium was in 1976 and 1977 at the LBNL. After several failed attempts by research groups in Russia, Germany, and the USA, livermorium was created successfully in 2000 at the Joint Institute for Nuclear Research by bombarding curium-248 atoms with calcium-48 atoms. The element was officially named livermorium in 2012.

Etymology

The name chalcogen comes from the Greek words χαλκος (chalkos, literally "copper"), and γενές (genes, born, gender, kindle). Thus the chalcogens bear copper. It was first used around 1930 by Wilhelm Biltz's group at the University of Hanover, where it was proposed by a man named Werner Fischer. Although the literal meanings of the Greek words imply that chalcogen means "copper-former", this is misleading because the chalcogens have nothing to do with copper in particular. "Ore-former" has been suggested as a better translation, wherefore both the vast majority of metal ores are chalcogenides and the word χαλκος in ancient Greek was associated with metals and metal-bearing rock in general; copper, and its alloy bronze, was one of the first metals to be used by humans.

Oxygen's name comes from the Greek words oxy genes, meaning "acid-forming". Sulfur's name comes from either the Latin word sulfurium or the Sanskrit word sulvere; both of those terms referred to sulfur in ancient times. Selenium is named after the Greek goddess of moon, Selene, to match the previously-discovered element tellurium, whose name comes from the Latin word telus, meaning earth. Polonium is named after Curie's country of birth, Poland. Livermorium is named for the Lawrence Livermore National Laboratory.

Occurrence

The four non-radioactive chalcogens at standard temperature and pressure.

The first four chalcogens (O, S, Se, and Te) are all primordial elements on Earth. Polonium forms naturally after the decay of other elements, even though it is not primordial.

Oxygen makes up 21% of the atmosphere by weight, 89% of water by weight, 46% of the earth's crust by weight, and 65% of the human body. Oxygen is also very common in minerals, being found in all oxide minerals and hydroxide minerals, and in numerous other mineral groups. Large stars (at least eight times the mass of the sun) also produce oxygen in their cores via nuclear fusion.

Sulfur makes up 0.035% of the earth's crust by weight, making it the 17th most abundant element there and makes up 0.25% of the human body. It is a major component of soil. Sulfur makes up 870 parts per million of seawater and about 1 part per billion of the atmosphere. Sulfur can be found in elemental form or in the form of sulfide minerals, sulfate minerals, or sulfosalt minerals. The largest stars (at least 12 times the mass of the sun) produce sulfur in their cores via nuclear fusion.

Selenium makes up 0.05 parts per million of the earth's crust by weight. This makes it the 67th most abundant element in the earth's crust. In the soil, selenium makes up on average 5 parts per million.Seawater consists of around 200 parts per trillion of selenium. The atmosphere consists of 1 nanogram of selenium per cubic meter. There exist mineral groups known as selenates and selenites, but there are not many of these. Selenium is not produced directly by nuclear fusion.

There are only 5 parts per billion of tellurium in the earth's crust and 15 parts per billion of tellurium in seawater. Tellurium is one of the eight or nine least abundant elements in the earth's crust. There are a few dozen tellurate minerals and telluride minerals.

Polonium only occurs in trace amounts on earth, via radioactive decay of uranium and thorium. It is present in uranium ores in concentrations of 100 micrograms per metric ton. Very minute amounts of polonium exist in the soil and thus in most food, and thus in the human body.

Livermorium is always produced artificially in nuclear reactors. Even when it is produced, only a small number of atoms at a time are synthesized.

Production

Roughly 100 million metric tons of oxygen are produced yearly. Oxygen is most commonly produced by cryogenic distillation, that is, cooling air until it becomes liquid, then extracting the liquid oxygen. Another method with which oxygen is produced is to send a stream of dry, clean air through a bed of molecular sieves made of zeolite, which absorbs the nitrogen in the air, leaving 90 to 93% pure oxygen.

Sulfur recovered from oil refining in Alberta, stockpiled for shipment in North Vancouver, B.C.

Sulfur can be mined in its elemental form, although this method is no longer as popular as it used to be. In 1865 a large deposit of elemental sulfur was discovered in the U.S. states of Louisiana and Texas, but it was difficult to extract at the time. In the 1890s, a man named Herman Frasch pondered the problem. Frasch came up with the solution of liquifying the sulfur with superheated steam and pumping the sulfur up to the surface. These days, however, sulfur is instead more often extracted from oil, natural gas, and tar.

The world production of selenium is around 1500 metric tons per year, out of which roughly 10% is recycled. Japan is the largest producer, producing 800 metric tons of selenium per year. Other large producers include Belgium (300 metric tons per year), the United States (thought to produce over 200 metric tons per year), Sweeden (130 metric tons per year), and Russia (100 metric tons per year). Selenium can be extracted from the waste from the process of electrolytically refining copper. Another method of producing selenium is to farm selenium-gathering plants such as milk vetch. This method could produce three kilograms of selenium per acre, but is not commonly practiced.

Tellurium is mostly produced as a by-product of the processing of copper. However, tellurium can also be refined by electrolytic reduction of sodium telluride. The world production of tellurium is between 150 and 200 metric tons per year. United States is one of the largest producers of tellurium, producing around 50 metric tons per year. Peru, Japan, and Canada are also large producers of tellurium.

Most isotopes of polonium are produced by bombarding bismuth with neutrons. However, some isotopes of polonium, ²¹⁰Po, ²¹¹Po, and ²¹²Po occur from the decay of various uranium and thorium isotopes.

All livermorium is produced artificially in nuclear reactors. The first successful production of livermorium was achieved by bombarding Curium-248 atoms with Calcium-48 atoms. As of 2011, roughly 25 atoms of livermorium have been synthesized.

Applications

Steelmaking is the most important use of oxygen; 55% of all oxygen produced goes to this application. The chemical industry also uses up numerous amounts of oxygen; 25% of all oxygen produced goes to this application. The remaining 20% of oxygen is mostly split between medical use, water treatment (as oxygen kills some types of bacteria), as rocket fuel (in liquid form), and for metal cutting.

A CdTe photovoltaic array

Most sulfur is transformed into sulfur dioxide, which is further transformed into sulfuric acid, a very common industrial chemical. Other common uses include being a key ingredient of gunpowder and being used to change soil pH. Sulfur is also mixed into rubber to vulcanize it.

Around 40% of all selenium produced goes to glassmaking. 30% of all selenium produced goes to metallurgy, including manganese production. 15% of all selenium produced goes to agriculture. Electronics, such as photovoltaic materials claim 10% of all selenium produced. Pigments account for 5% of all selenium produced. Historically, machines such as photocopiers and light meters used one-third of all selenium produced, but this application has been in steady decline.

Tellurium suboxide is used in the rewritable data layer of some CD-RW disks and DVD-RW disks. Bismuth telluride is also present in many microelectronic devices, such as photoreceptors. Tellurium is sometimes used as an alternative to sulfur in vulcanized rubber. Cadmium telluride is a high-efficiency material in solar panels.

Some of polonium's applications relate to the element's radioactivity. Most polonium, however, is used in antistatic devices. Polonium is used as an alpha-particle generator for research. Polonium alloyed with beryllium provides an efficient neutron source. Polonium is also used in nuclear batteries.

Biological role

Oxygen is needed by almost all organisms for the purpose of generating ATP. Also, oxygen is a key component of most biological compounds, such as water and DNA.

All animals need significant amounts of sulfur. Some amino acids, such as cystine and methionine contain sulfur. Plant roots take up sulfate ions from the soil and reduce it to sulfide ions. Metalloproteins also use sulfur to attach to useful metal atoms in the body and sulfur similarly attaches itself to poisonous metal atoms like cadmium to haul them to the safety of the liver. On average, humans consume 900 milligrams of sulfur each day. Sulfur compounds, such as those found in skunk spray often have strong odors.

All animals need trace amounts of selenium, but only for some specialized enzymes. Humans consume on average between 6 and 200 micrograms of selenium per day. Mushrooms and brazil nuts are especially noted for their high selenium content. Selenium in foods is most commonly found in the form of amino acids such as selenocysteine and selenomethionine.

Tellurium is not known to be needed for animal life, although a few fungi can incorporate it in compounds in place of selenium. Microorganisms also absorb tellurium and emit dimethyl telluride. Most tellurium in the blood stream is excreted slowly in urine, but some is converted to dimethyl telluride and released through the lungs. On average, humans ingest about 600 micrograms of tellurium daily. Plants can take up some tellurium from the soil. Onions and garlic have been found to contain as much as 300 parts per million in dry weight.

Polonium is highly toxic on account of being radioactive. As little as 10 nanograms of polonium can be lethal. Livermorium has not been produced in high enough amounts for its role in organisms to be detectable.

Toxicity

Oxygen is generally nontoxic. In fact, in both elemental gaseous form and as a component to water, it is vital to almost all life on earth. Liquid oxygen, however, is "life-threateningly fierce". Even gaseous oxygen is dangerous in excess. For instance, sports divers have occasionally drowned from convulsions caused by breathing pure oxygen at a depth of more than 10 meters (33 feet) underwater. Ozone, an allotrope of oxygen, is toxic to most life. It can cause lesions in the respiratory tract.

Sulfur is generally nontoxic; it is even a vital nutrient for humans. However, many of its compounds, such as Hydrogen sulfide (H₂S) and Sulfur dioxide (SO₂) are highly toxic.

Selenium is a trace nutrient, required by humans on the order of tens or hundreds of micrograms per day. However, a dose of over 450 micrograms can be toxic, resulting in symptoms of bad breath and body odor. Extended, low-level exposure, which can occur at some industries, weight loss, anemia, and dermatitis. Hydrogen selenide (H₂Se) is highly toxic.

Tellurium is not generally highly toxic. However, as little as 10 micrograms of tellurium per cubic meter of air can cause notoriously unpleasant breath, described as smelling like rotten garlic. Acute tellurium poisoning can cause vomiting, gut inflammation, internal bleeding, and respiratory failure. Extended, low-level exposure to tellurium causes tiredness and indigestion. Sodium tellurite (Na₂TeO₃) is lethal in amounts of around 2 grams.

Polonium-210 is, by weight, a billion times as toxic as hydrogen cyanide. It has been used as a murder weapon in the past.

See also

• Gold chalcogenides