A chalcogen is any element belonging to Group 16 of the periodic table: oxygen, sulfur, selenium, tellurium, polonium, and the synthetic element livermorium. Sometimes called the “oxygen family,” this group includes some of the most abundant and biologically critical elements on Earth, along with some of the rarest and most radioactive.
The Six Chalcogens
The name “chalcogen” comes from Greek roots meaning “ore former,” which reflects how often these elements appear in metal ores found in the Earth’s crust. Oxygen alone makes up about 46.6% of the crust by weight, making it the single most abundant element in the rocks and soil beneath your feet. Sulfur accounts for roughly 0.12%. Selenium and tellurium are far scarcer, and polonium is radioactive with no stable isotopes. Livermorium, the sixth member, was first synthesized in 2000 at a nuclear research lab in Dubna, Russia, by bombarding curium atoms with calcium ions. Its most stable isotope lasts only about 53 milliseconds before decaying.
What Makes Them a Family
Every chalcogen has six electrons in its outermost energy level, arranged in what chemists describe as an ns²np⁴ configuration. That means each one is just two electrons short of a completely filled outer shell. This shared trait gives the group a common chemical personality: they readily pick up two extra electrons from metals, forming compounds with a -2 charge. Table salt’s cousin, for instance, is replaced by metal sulfides and metal oxides following the same basic logic.
The heavier chalcogens (sulfur, selenium, tellurium) can also lose electrons instead of gaining them, reaching +4 or +6 charge states. Oxygen almost never does this because it holds onto its electrons so tightly. That grip is reflected in electronegativity values that drop steadily down the group: 3.4 for oxygen, 2.6 for sulfur, 2.6 for selenium, 2.1 for tellurium, and 2.0 for polonium.
Physical Properties and Allotropes
The chalcogens span an enormous range of physical states at room temperature. Oxygen is a colorless gas that boils at a frigid 90 K (about -183°C). Sulfur is a brittle yellow solid that doesn’t boil until 718 K (about 445°C). Selenium boils at 958 K, tellurium at 1,261 K, and polonium at 1,235 K. The trend from gas to increasingly high-boiling solids reflects the growing size and stronger interactions between heavier atoms.
Several chalcogens exist in multiple structural forms, called allotropes. Oxygen is most familiar as the two-atom molecule (O₂) you breathe, but it also forms ozone (O₃), a three-atom version that absorbs ultraviolet radiation in the upper atmosphere. Sulfur is even more varied. Its most common form at room temperature is rhombic sulfur, built from rings of eight sulfur atoms packed into crystals. Warming it to about 95.5°C converts it to monoclinic sulfur, a different crystal arrangement of the same eight-atom rings. At high temperatures in gas form, sulfur breaks down into smaller clusters of six, four, and ultimately two atoms. Selenium has a stable gray form along with two red allotropes, one of which mirrors sulfur’s eight-atom ring structure.
Roles in Living Organisms
Three chalcogens are essential to life. Oxygen’s role is the most obvious: it drives the energy-producing reactions in nearly every cell of your body. Without a steady supply, cells can’t efficiently extract energy from food.
Sulfur is built into two of the amino acids that make up proteins. It also shows up in iron-sulfur clusters, tiny molecular machines inside enzymes that shuttle electrons during chemical reactions. These clusters are ancient structures found across nearly all forms of life.
Selenium plays a more specialized but equally important role. Your body incorporates it into proteins as selenocysteine, sometimes called the 21st amino acid. Selenium-containing enzymes protect cells from oxidative damage by breaking down hydrogen peroxide and other reactive molecules that would otherwise harm DNA and cell membranes. Selenium is also necessary for converting the thyroid hormone thyroxine (T4) into its active form, triiodothyronine (T3), which regulates metabolism, growth, and development. A selenium deficiency can therefore affect both your antioxidant defenses and your thyroid function.
Industrial and Commercial Uses
Sulfur’s biggest industrial role is in sulfuric acid production, one of the most widely manufactured chemicals in the world. It’s used in fertilizers, metal processing, and petroleum refining. Sulfur is also the classic vulcanizing agent for rubber, creating the cross-links between polymer chains that make tires durable and elastic.
Tellurium has become increasingly important in renewable energy. About 40% of global tellurium consumption goes into cadmium telluride thin-film solar cells, a leading technology for utility-scale solar power. Another 30% is used in thermoelectric devices that convert heat directly into electricity. The remaining demand splits among metallurgy (15%), rubber compounding (5%), and other applications (10%). Selenium can substitute for tellurium in some rubber vulcanization processes.
Chalcogenide glasses, made by combining sulfur, selenium, or tellurium with elements like germanium or arsenic, are transparent to infrared light in ways that ordinary glass is not. This makes them valuable for infrared optics, thermal imaging lenses, and fiber-optic components designed for wavelengths beyond the visible spectrum. Their sensitivity to light also makes them useful in rewritable optical storage media. When doped with rare-earth elements, these glasses can amplify infrared signals, opening up applications in optical communication devices.
Toxicity Across the Group
The chalcogens range from life-sustaining to lethal depending on which element you’re dealing with and in what quantity. Oxygen is obviously essential, but even it becomes toxic at high concentrations and pressures. Sulfur compounds like hydrogen sulfide are poisonous at relatively low airborne concentrations, though elemental sulfur itself is comparatively mild.
Selenium sits in an unusually narrow window between necessity and toxicity. Your body needs trace amounts for normal enzyme function, but intake beyond a few hundred micrograms per day can cause selenosis, with symptoms like hair loss, nausea, and nerve damage. Tellurium exposure, even in small amounts, causes a distinctive garlic-like odor in breath and sweat as the body converts it to dimethyl telluride. Occupational exposure limits exist for tellurium dust and fumes, though data on its most dangerous compound, hydrogen telluride, remains sparse.
Polonium is in a different category entirely. As an intensely radioactive alpha emitter, polonium-210 is one of the most toxic substances known when ingested or inhaled. Microgram quantities can be fatal, which is why it gained notoriety as a poison in the 2006 assassination of Alexander Litvinenko in London.