- Sulfur (S) is a yellow, nonmetallic element with atomic number 16, positioned in Group 16 of the periodic table.
- It has six valence electrons in its outermost shell, giving it the ability to form compounds in oxidation states from –2 to +6. Its atomic structure consists of sixteen protons, typically sixteen neutrons, and sixteen electrons arranged in three shells (two in the first shell, eight in the second, and six in the third).
- Sulfur has four stable isotopes: sulfur-32 (³²S), which makes up about 94.99% of natural sulfur; sulfur-33 (³³S), about 0.75%; sulfur-34 (³⁴S), about 4.25%; and sulfur-36 (³⁶S), about 0.01%.
- In its most common form, sulfur exists as bright yellow orthorhombic crystals composed of S₈ rings, which are stable at room temperature.
- Sulfur is the tenth most abundant element in the universe and is found widely on Earth in both elemental form and in numerous compounds. It occurs in volcanic deposits, hot springs, and sedimentary evaporite minerals such as gypsum (CaSO₄·2H₂O) and anhydrite (CaSO₄). It is also found in sulfide minerals like pyrite (FeS₂), galena (PbS), and sphalerite (ZnS). Large underground deposits of elemental sulfur can be extracted by the Frasch process, which uses superheated water to melt the sulfur for pumping to the surface. Modern sulfur production also comes largely from the purification of petroleum and natural gas, where hydrogen sulfide (H₂S) is removed and converted into elemental sulfur via the Claus process.
- Sulfur has been known since antiquity and was recognized in ancient times as brimstone. References to it appear in biblical texts and ancient civilizations’ writings. Its name comes from the Latin sulphur, meaning “to burn,” reflecting its flammability and the suffocating odor of its combustion products.
- Biologically, sulfur is essential for life. It is a component of certain amino acids (cysteine and methionine) and vitamins (biotin and thiamine) and is crucial for protein structure through disulfide bonds. Sulfate ions are involved in metabolic processes and are found in all living cells. In the environment, sulfur is a key part of the biogeochemical sulfur cycle, moving between the atmosphere, lithosphere, and biosphere.
- Industrially, sulfur is a major raw material for producing sulfuric acid (H₂SO₄), one of the most important industrial chemicals, used in fertilizer production, petroleum refining, mineral processing, and chemical manufacturing. Sulfur is also used in vulcanization of rubber, production of fungicides and insecticides, and in the manufacture of batteries and detergents.
- Chemically, sulfur forms a wide variety of compounds, including sulfides, sulfates, and sulfur oxides. It can exist in multiple allotropes and oxidation states, enabling rich chemistry. Burning sulfur in air produces sulfur dioxide (SO₂), a pungent gas that can cause respiratory problems and contribute to acid rain when oxidized further to sulfur trioxide (SO₃) and dissolved in water to form sulfuric acid.
- While sulfur is non-toxic in its elemental form, many sulfur compounds, such as hydrogen sulfide (H₂S), are highly toxic and corrosive. Environmental regulation has reduced emissions of sulfur oxides from industrial sources to mitigate acid rain and air pollution.
