The Chalcogens (Group 16): Oxygen, Sulfur, Selenium, Tellurium and Polonium

Group 16 of the periodic table — oxygen, sulfur, selenium, tellurium, polonium and the superheavy livermorium — is known as the chalcogens, from Greek words meaning “ore-formers”. Many metal ores are oxides or sulfides, so these elements are where much of the Earth’s metal is found.

Six outer electrons

Every chalcogen has six outer electrons (ns² np⁴), two short of a full octet. They tend to gain two electrons to form −2 ions (oxide O²⁻, sulfide S²⁻) or form two covalent bonds, as oxygen does in water. Below oxygen, they can also use more of their electrons, reaching +4 and +6 — sulfur is +6 in sulfuric acid.

The group at a glance (from our element data)

Element	Type	State at 25 °C	Melting point (°C)	Electronegativity
Oxygen	Nonmetal	Gas (O₂)	−218.8	3.44
Sulfur	Nonmetal	Solid	115.2	2.58
Selenium	Nonmetal	Solid	220.5	2.55
Tellurium	Metalloid	Solid	449.5	2.1
Polonium	Metal (or metalloid)	Solid	254	2.0

Like the neighbouring groups, the chalcogens become more metallic going down.

Oxygen

Oxygen is the most abundant element in the Earth’s crust and in the human body by mass (see elements in the human body), and about 21% of the air. It’s the second most electronegative element, which makes it a powerful oxidizing agent — hence combustion, rusting and respiration. See oxidation and reduction.

Oxygen also exists as ozone (O₃), an allotrope with three atoms. In the stratosphere, the ozone layer absorbs harmful ultraviolet radiation; near the ground, ozone is a lung-irritating pollutant.

Liquid oxygen is pale blue and is attracted to a magnet, because the O₂ molecule has two unpaired electrons.

Sulfur

Sulfur — “brimstone” in older texts — is a yellow, brittle solid found near volcanoes and extracted from natural gas and oil during refining. It forms rings of eight atoms (S₈) and has more allotropes than any other element.

Its most important compound by far is sulfuric acid, made in larger quantities than any other chemical and used for fertilizers, batteries, and processing ores. Sulfur also:

• appears in two amino acids and in the disulfide bridges that give hair and proteins their shape;
• is responsible for many bad smells — hydrogen sulfide (rotten eggs), the compounds in skunk spray, and the smell of cooked cabbage;
• was used to vulcanize rubber, making it durable enough for tyres;
• forms sulfur dioxide when fuels containing it burn — a cause of acid rain.

Selenium

Selenium is an essential trace nutrient — part of antioxidant enzymes — but toxic in larger amounts, with a narrow gap between the two. Its electrical conductivity increases in light, which led to its use in early photocells and photocopiers. Today it’s used in glassmaking (to remove green tints or make red glass) and some solar cells.

Tellurium

Tellurium is one of the rarest stable elements in the Earth’s crust. It’s used in cadmium telluride thin-film solar panels and in thermoelectric devices that turn heat into electricity. Exposure gives people breath that smells strongly of garlic, a notorious hazard for anyone working with it.

Polonium

Polonium was discovered by Marie and Pierre Curie in 1898 and named after Marie’s homeland, Poland. It’s intensely radioactive: polonium-210 is an alpha emitter so powerful that a sample glows blue from ionizing the air around it and stays warm from its own decay energy. It’s used in antistatic devices, and it was infamously used to poison Alexander Litvinenko in 2006. See Marie Curie’s elements.

Quick answers

Why is the group called chalcogens? From Greek, roughly “ore-formers”, since many metals occur as oxides and sulfides.

Is sulfur a metal? No — it’s a nonmetal. Tellurium is a metalloid, and polonium is usually classed as a metal.

Why does oxygen form O₂ but sulfur forms S₈? Oxygen forms strong double bonds, so O=O is favoured. Sulfur forms weaker double bonds and prefers single bonds to two neighbours, making rings and chains.