Chalcogen

The chalcogens are the chemical elements in group 16 of the periodic table. This group is also known as the oxygen family. Group 16 consists of the elements oxygen, sulfur, selenium, tellurium, and the radioactive elements polonium and livermorium. Often, oxygen is treated separately from the other chalcogens, sometimes even excluded from the scope of the term "chalcogen" altogether, due to its very different chemical behavior from sulfur, selenium, tellurium, and polonium. The word "chalcogen" means "ore-forming"; chalcogens got their name because protoscientists and early scientists could discern that these essences were involved in ore formation.
Sulfur has been known since antiquity, and oxygen was recognized as an element in the 18th century. Selenium, tellurium and polonium were discovered in the 19th century, and livermorium in 2000. All of the chalcogens have six valence electrons, leaving them two electrons short of a full outer shell. Their most common oxidation states are −2, +2, +4, and +6. They have relatively small atomic radii, especially the lighter ones.
All of the naturally occurring chalcogens have some role in biological functions, either as a nutrient or a toxin. Selenium is an important nutrient but is also commonly toxic. Tellurium often has unpleasant effects, and polonium is always harmful as a result of its radioactivity.
Sulfur has more than 20 allotropes, oxygen has nine, selenium has at least eight, polonium has two, and only one crystal structure of tellurium has so far been discovered. There are numerous organic chalcogen compounds. Not counting oxygen, organic sulfur compounds are generally the most common, followed by organic selenium compounds and organic tellurium compounds. This trend also occurs with chalcogen pnictides and compounds containing chalcogens and carbon group elements.
Oxygen is generally obtained by separation of air into nitrogen and oxygen. Sulfur is extracted from oil and natural gas. Selenium and tellurium are produced as byproducts of copper refining. Polonium is most available in naturally occurring actinide-containing materials. Livermorium has been synthesized in particle accelerators. The primary use of elemental oxygen is in steelmaking. Sulfur is mostly converted into sulfuric acid, which is heavily used in the chemical industry. Selenium's most common application is glassmaking. Tellurium compounds are mostly used in optical disks, electronic devices, and solar cells. Some of polonium's applications are due to its radioactivity.

Properties

Atomic and physical

Chalcogens show similar patterns in electron configuration, especially in the outermost shells, where they all have the same number of valence electrons, resulting in similar trends in chemical behavior:

Z	Element	Electrons per 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

Element	Melting point	Boiling point	Density at STP
Oxygen	−219	−183	0.00143
Sulfur	120	445	2.07
Selenium	221	685	4.3
Tellurium	450	988	6.24
Polonium	254	962	9.2
Livermorium	364–507	762–862	14

All chalcogens have six valence electrons. All of the solid, stable chalcogens are soft and do not conduct heat well. Electronegativity decreases towards the chalcogens with higher atomic numbers. Density, melting and boiling points, and atomic and ionic radii tend to increase towards the chalcogens with higher atomic numbers.

Isotopes

Out of the six known chalcogens, one has an atomic number equal to a nuclear magic number, which means that their atomic nuclei tend to have increased stability against radioactive decay. Oxygen has three stable isotopes, and 14 unstable ones. Sulfur has four stable isotopes, 20 radioactive ones, and one isomer. Selenium has six observationally stable or nearly stable isotopes, 26 radioactive isotopes, and 9 isomers. Tellurium has eight stable or nearly stable isotopes, 31 unstable ones, and 17 isomers. Polonium has 42 isotopes, none of which are stable. It has an additional 28 isomers. In addition to the stable isotopes, some radioactive chalcogen isotopes occur in nature, either because they are decay products, such as ²¹⁰Po, because they are primordial, such as ⁸²Se, because of cosmic ray spallation, or via nuclear fission of uranium. Livermorium isotopes ²⁸⁸Lv through ²⁹³Lv have been discovered; the most stable livermorium isotope is ²⁹³Lv, which has a half-life of 0.061 seconds.
With the exception of livermorium, all chalcogens have at least one naturally occurring radioisotope: oxygen has trace ¹⁵O, sulfur has trace ³⁵S, selenium has ⁸²Se, tellurium has ¹²⁸Te and ¹³⁰Te, and polonium has ²¹⁰Po.
Among the lighter chalcogens, the most neutron-poor isotopes undergo proton emission, the moderately neutron-poor isotopes undergo electron capture or β⁺ decay, the moderately neutron-rich isotopes undergo β⁻ decay, and the most neutron rich isotopes undergo neutron emission. The middle chalcogens have similar decay tendencies as the lighter chalcogens, but no proton-emitting isotopes have been observed, and some of the most neutron-deficient isotopes of tellurium undergo alpha decay. Polonium isotopes tend to decay via alpha or beta decay. Isotopes with nonzero nuclear spins are more abundant in nature among the chalcogens selenium and tellurium than they are with sulfur.

Allotropes

Oxygen's most common allotrope is diatomic oxygen, or O₂, a reactive paramagnetic molecule that is ubiquitous to aerobic organisms and has a blue color in its liquid state. Another allotrope 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 including "red oxygen", which has the formula O₈.
Sulfur has over 20 known allotropes, which is more than any other element except carbon. The most common allotropes are in the form of eight-atom rings, but other molecular allotropes that contain as few as two atoms or as many as 20 are known. Other notable sulfur allotropes include 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 °C, the chains begin to break down. If liquid sulfur above 190 °C is frozen very rapidly, the resulting sulfur is amorphous or "plastic" sulfur. Gaseous sulfur is a mixture of diatomic sulfur and 8-atom rings.
Selenium has at least eight distinct 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. 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. The red allotrope converts to the black allotrope in the presence of heat. The gray allotrope of selenium is made from spirals on selenium atoms, while one of the red allotropes is made of stacks of selenium rings.
Tellurium is not known to have any allotropes, although its typical form is hexagonal. Polonium has two allotropes, which are known as α-polonium and β-polonium. α-polonium has a cubic crystal structure and converts to the rhombohedral β-polonium at 36 °C.
The chalcogens have varying crystal structures. Oxygen's crystal structure is monoclinic, sulfur's is orthorhombic, selenium and tellurium have the hexagonal crystal structure, while polonium has a cubic crystal structure.

Chemical

Oxygen, sulfur, and selenium are nonmetals, and tellurium is a metalloid, meaning that its chemical properties are between those of a metal and those of a nonmetal. It is not certain whether polonium is a metal or a metalloid. Some sources refer to polonium as a metalloid, although it has some metallic properties. Also, some allotropes of selenium display characteristics of a metalloid, even though selenium is usually considered a nonmetal. Even though oxygen is a chalcogen, its chemical properties are different from those of other chalcogens. One reason for this is that the heavier chalcogens have vacant d-orbitals. Oxygen's electronegativity is also much higher than those of the other chalcogens. This makes oxygen's electric polarizability several times lower than those of the other chalcogens.
For covalent bonding a chalcogen may accept two electrons according to the octet rule, leaving two lone pairs. When an atom forms two single bonds, they form an angle between 90° and 120°. In 1+ cations, such as hydroxonium|, a chalcogen forms three molecular orbitals arranged in a trigonal pyramidal fashion and one lone pair. Double bonds are also common in chalcogen compounds, for example in chalcogenates.
The oxidation number of the most common chalcogen compounds with positive metals is −2. However the tendency for chalcogens to form compounds in the −2 state decreases towards the heavier chalcogens. Other oxidation numbers, such as −1 in pyrite and peroxide, do occur. The highest formal oxidation number is +6. This oxidation number is found in sulfates, selenates, tellurates, polonates, and their corresponding acids, such as sulfuric acid.
Oxygen is the most electronegative element except for fluorine, and forms compounds with almost all of the chemical elements, including some of 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 relatively common. With hydrogen it forms water and hydrogen peroxide. Organic oxygen compounds are ubiquitous in organic chemistry.
Sulfur's oxidation states are −2, +2, +4, and +6. Sulfur-containing analogs of oxygen compounds often have the prefix thio-. Sulfur's chemistry is similar to oxygen's, in many ways. One difference is that sulfur-sulfur double bonds are far weaker than oxygen-oxygen double bonds, but sulfur-sulfur single bonds are stronger than oxygen-oxygen single bonds. Organic sulfur compounds such as thiols have a strong specific smell, and a few 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 tellurium monoxide, tellurium dioxide, and tellurium trioxide. Polonium's oxidation states are +2 and +4.
There are many acids containing chalcogens, including sulfuric acid, sulfurous acid, selenic acid, and telluric acid. All hydrogen chalcogenides are toxic except for water. Oxygen ions often come in the forms of oxide ions, peroxide ions, and hydroxide ions. Sulfur ions generally come in the form of sulfides, bisulfides, sulfites, sulfates, and thiosulfates. Selenium ions usually come in the form of selenides, selenites and selenates. Tellurium ions often come in the form of tellurates. Molecules containing metal bonded to chalcogens are common as minerals. For example, pyrite is an iron ore, and the rare mineral calaverite is the ditelluride.
Although all group 16 elements of the periodic table, including oxygen, can be 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.
Except for polonium, the chalcogens are all fairly similar to each other chemically. They all form X²⁻ ions when reacting with electropositive metals.
Sulfide minerals and analogous compounds produce gases upon reaction with oxygen.