Sulfur

Sulfur is a bright yellow solid that has been known since ancient times as brimstone: the fiery, smelly substance of biblical descriptions of fire and punishment. Today it is used to make the most produced industrial chemical in the world, to vulcanise rubber for car tyres, and to make the gunpowder that powered warfare for nearly 1,000 years.

  • Atomic Number1616 protons, 16 electrons
  • Atomic Mass32.07 uAbout 32× heavier than hydrogen
  • State at Room TempSolidbright yellow solid
  • Density2.067 g/cm³About twice as dense as water
  • Melting / Boiling115.2°C / 444.6°CMelts at 113°C, boils at 445°C
  • DiscoveredAncientKnown since ancient times

How does sulfur compare to the other chalcogens?

Sulfur is the lightest solid member of Group 16. See how it compares in mass.

Atomic Mass Comparison
Oxygen16 u
Sulfur32 u
Selenium79 u
Iron56 u
Tellurium128 u

Sulfur at 32 atomic mass units is exactly twice the mass of oxygen (16 u). Selenium (79 u) is about two and a half times heavier than sulfur. Despite these mass differences, all Group 16 elements tend to form two bonds.

What is sulfur?

Sulfur is a non-metal in Group 16 of the periodic table. At room temperature it forms yellow crystals made of rings of eight sulfur atoms (S₈). It does not conduct electricity and has a brittle, waxy texture. When burned in air, sulfur produces sulfur dioxide (SO₂), a sharp-smelling gas that dissolves in rainwater to form sulfurous acid: one of the main causes of acid rain.

The word sulfur comes from the Latin sulphur, possibly related to the Sanskrit word ulvana. In medieval times it was called brimstone (burning stone) because it catches fire easily, producing a vivid blue flame and a choking smell. The Bible uses brimstone as a symbol of divine punishment, reflecting how alarming its burning smell and volcanic origins seemed to ancient peoples.

Fact Sulfuric acid (H₂SO₄) is the most produced industrial chemical in the world by a wide margin, global production exceeds 250 million tonnes per year. The quantity produced is so large that economists sometimes use sulfuric acid output as an indicator of a country's industrial activity and economic health.

Where you find sulfur

In space

Sulfur is particularly abundant on Io, one of Jupiter's moons. Io is the most volcanically active body in the solar system, and its surface is coated in orange, yellow and white deposits of sulfur and sulfur compounds from its hundreds of active volcanoes. Venus has thick clouds of sulfuric acid. Sulfur is found in the Sun and most other stars.

On Earth

Sulfur is found throughout the Earth's crust, both as a pure element and in many compounds.

  • Volcanic deposits. Pure sulfur forms around volcanic vents and hot springs where hydrogen sulfide gas reacts with air. The Kawah Ijen volcano in Indonesia has spectacular sulfur mining, workers carry glowing blue sulfur flames at night.
  • Sulfide ores. Iron pyrite (fool's gold, FeS₂), galena (lead sulfide) and chalcopyrite (copper-iron sulfide) are major metal ores that also provide sulfur.
  • Fossil fuels. Oil, coal and natural gas contain sulfur compounds that must be removed before burning to prevent acid rain.

How we use sulfur

  • Sulfuric acid. More sulfuric acid (H₂SO₄) is produced than any other chemical in the world. It is used to make fertilisers, car batteries, detergents, dyes and dozens of other products.
  • Vulcanised rubber. Adding sulfur to natural rubber and heating it, vulcanisation, invented by Charles Goodyear in 1839, turns it into the tough, elastic material used in car tyres, shoe soles and rubber bands.
  • Gunpowder. Traditional black gunpowder is a mixture of potassium nitrate, charcoal and sulfur. Sulfur helps the powder ignite more easily and burn more consistently, used in weapons and fireworks for nearly 1,000 years.
  • Fungicides and pesticides. Sulfur powder is one of the oldest pesticides still in use, sprayed on crops to kill fungi. It is approved for organic farming because it is a natural element.
Did you know? The rotten-egg smell of bad eggs, swamps and volcanoes is not sulfur itself, pure sulfur is almost odourless. The smell comes from hydrogen sulfide (H₂S), a gas produced when bacteria decompose organic matter containing sulfur. Your body makes tiny amounts of hydrogen sulfide as a signalling molecule inside your own cells.

How it was discovered

Sulfur has been known since prehistoric times. It occurs naturally near volcanoes as bright yellow deposits that were easy to collect. Ancient Egyptians used sulfur dioxide to bleach cloth. The Romans used burning sulfur as a fumigant and a weapon of siege warfare. In China, gunpowder containing sulfur was in use by the 9th century CE. Sulfur's elemental nature, that it was a single chemical substance, not a compound, was established by Antoine Lavoisier in 1789 as part of his revolutionary rethinking of chemistry.

Deeper dive: sulfuric acid production and the contact process

The contact process is the industrial method for making sulfuric acid. Sulfur is burned to produce sulfur dioxide (SO₂), which is oxidised to sulfur trioxide (SO₃) over a vanadium pentoxide catalyst at around 450°C. The SO₃ is absorbed into concentrated sulfuric acid to produce oleum, then diluted. The process converts over 99.7% of the sulfur. Without sulfuric acid, modern agriculture, and thus modern civilisation, would be impossible, as it is needed to make phosphate and nitrogen fertilisers.

Sulfur plays a crucial role in protein structure. The amino acids cysteine and methionine both contain sulfur atoms. The sulfur bridges between cysteine molecules help give proteins their three-dimensional shapes. Keratin: the protein in your hair, nails and skin, is cross-linked by sulfur bridges. Curly hair has more sulfur cross-links than straight hair; perming applies chemicals that break and reform these bridges to permanently change hair shape.

Acid rain is formed when sulfur dioxide (from burning coal and oil) or nitrogen oxides dissolve in rainwater to form dilute sulfuric and nitric acid. Acid rain harms forests, acidifies lakes (killing fish) and erodes limestone buildings and statues. Stricter regulations on sulfur emissions from power stations and ships have significantly reduced acid rain in Europe and North America since the 1980s.

Sulfur is an ancient element that underlies modern industry, from fertilisers to rubber to batteries. Moving to 17 protons brings us to chlorine, the halogen that purifies swimming pools and water supplies.