Sulfur (also spelled sulphur in some forms of British English) is the chemical element with symbol S and atomic number 16. It is a yellow, brittle nonmetal in group 16 of the periodic table and occurs naturally in elemental deposits, sulfide minerals and sulfate minerals.
Sulfur is essential to living organisms and is also one of the most important industrial elements because enormous quantities are converted into sulfuric acid, a foundational chemical used in fertilizers, metal processing, petroleum refining and many manufacturing processes.
Sulfur quick facts
| Property | Value |
| Symbol | S |
| Atomic number | 16 |
| Relative atomic mass | 32.06 |
| Group | 16 |
| Period | 3 |
| Block | p-block |
| Classification | Nonmetal / chalcogen |
| State at 20°C | Solid |
| Density | 2.07 g/cm³ |
| Melting point | 115.21°C |
| Boiling point | 444.61°C |
| Electron configuration | [Ne] 3s² 3p⁴ |
| Electronegativity | 2.58 (Pauling scale) |
| CAS number | 7704-34-9 |
| Main natural isotope | ³²S |
Where is sulfur on the periodic table?
Sulfur is in group 16, period 3 and the p-block. Its electron configuration is [Ne] 3s² 3p⁴, giving it six valence electrons. Common oxidation states include −2, +4 and +6.
See the complete CleverlySmart periodic table. Sulfur lies directly below oxygen in group 16.
Physical properties and allotropes
The most familiar form of sulfur is a bright yellow crystalline solid. Sulfur has several allotropes, many built from ring-shaped S₈ molecules. Rhombic sulfur is the stable form near room temperature, while monoclinic sulfur is stable over a higher temperature range before melting.
Sulfur is a poor conductor of electricity and heat compared with metals. When molten sulfur is heated, its viscosity can change dramatically as sulfur rings open and form long chains.
Chemical properties of sulfur
Sulfur reacts with many metals to form sulfides and burns in oxygen to form sulfur dioxide:
S + O₂ → SO₂
Sulfur dioxide can be further oxidised to sulfur trioxide, which is used in the manufacture of sulfuric acid. Sulfur also forms compounds with hydrogen, halogens and many other elements.
Important sulfur compounds
- Sulfuric acid (H₂SO₄): one of the world’s highest-volume industrial chemicals, used in fertilizers, batteries, metal treatment, chemical manufacture and petroleum processing.
- Sulfur dioxide (SO₂): used in chemical manufacture and as a preservative in some applications; it is also an important air pollutant when released by combustion.
- Hydrogen sulfide (H₂S): a highly toxic, flammable gas with the characteristic smell of rotten eggs at low concentrations.
- Sulfates: salts containing SO₄²⁻, found in minerals and used widely in industry.
- Sulfides: compounds containing sulfur in a low oxidation state; many important metal ores are sulfide minerals.
Where sulfur comes from
Native sulfur occurs in volcanic regions and certain sedimentary deposits, but much of today’s sulfur is recovered as a by-product of oil and natural-gas processing. Hydrogen sulfide is removed from fuels and converted to elemental sulfur, reducing sulfur emissions when the fuels are burned.
Uses of sulfur
- Sulfuric acid production: by far the largest industrial use.
- Fertilizers: sulfuric acid is used to manufacture phosphate fertilizers, while elemental sulfur and sulfate fertilizers can supply sulfur nutrients to crops.
- Rubber vulcanisation: sulfur cross-links polymer chains, improving strength, elasticity and durability.
- Agriculture: elemental sulfur is used in some fungicides and soil treatments.
- Chemical manufacturing: sulfur compounds are used in dyes, detergents, pharmaceuticals, paper processing and many other products.
Biological role of sulfur
Sulfur is an essential element for life. It occurs in the amino acids cysteine and methionine, in several vitamins and cofactors, and in iron-sulfur proteins. Disulfide bonds between sulfur atoms in proteins can help stabilise three-dimensional protein structures.
Sulfur isotopes
Natural sulfur contains four stable isotopes: ³²S, ³³S, ³⁴S and ³⁶S. Sulfur-32 is the most abundant. Variations in sulfur isotope ratios are important tools in geology, environmental science, archaeology and studies of biogeochemical cycles.
For other stable and radioactive nuclides, see the CleverlySmart complete isotope table.
Safety and environmental concerns
Elemental sulfur is combustible and sulfur dust can create fire or explosion hazards. Several sulfur compounds require much greater caution: hydrogen sulfide is acutely toxic, and sulfur dioxide can irritate the respiratory system. Burning sulfur-containing fuels can contribute to sulfur oxide emissions and acid deposition unless emissions are controlled.
History and name
Sulfur has been known since antiquity and was used in medicines, fumigation and early pyrotechnic mixtures. Because it was known long before modern chemistry, it has no single modern discoverer. The traditional term brimstone is an old name for sulfur.
Frequently asked questions
What colour is sulfur?
The common crystalline form is bright yellow.
Does sulfur smell like rotten eggs?
Pure elemental sulfur is essentially odourless. The familiar rotten-egg smell comes mainly from hydrogen sulfide (H₂S), not sulfur itself.
What is sulfur mainly used for?
Most sulfur is used to manufacture sulfuric acid.
Sulfur allotropes and molecular structure
Elemental sulfur commonly occurs as rings of eight sulfur atoms, S₈. The most familiar crystalline forms are rhombic and monoclinic sulfur. Heating can open the S₈ rings and create long sulfur chains, which is why molten sulfur becomes unusually viscous over part of its temperature range.
Sulfuric acid and industrial importance
The most important sulfur compound by industrial scale is sulfuric acid (H₂SO₄). It is manufactured mainly by the Contact process and is used in fertiliser production, mineral processing, petroleum refining, batteries and many chemical syntheses. Sulfur itself is also used to vulcanise rubber, improving elasticity and durability by creating cross-links between polymer chains.
Sulfur in nature
Sulfur cycles continuously through rocks, oceans, the atmosphere and living organisms. Sulfur-containing amino acids such as cysteine and methionine are essential parts of proteins. In the environment, sulfur can occur as sulfides, sulfates and elemental sulfur, while volcanic gases commonly contain sulfur dioxide and hydrogen sulfide.
Frequently asked questions
Why does sulfur sometimes smell?
Pure elemental sulfur has little smell. The familiar “sulfur smell” usually comes from volatile sulfur compounds such as hydrogen sulfide, thiols or sulfur dioxide.
Is sulfur essential to life?
Yes. Sulfur is required in important amino acids, vitamins and enzyme cofactors.
