Atomic number 16

Sulfur

S
+ phase
− phase

Shells Orbitals 3s 3p

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16 electrons in 3 shells: 2, 8, 6.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Nonmetal
Period: 3
Group: 16
Block: p
Standard atomic weight: 32.06(2) Da
Electron configuration: [Ne] 3s² 3p⁴
Phase at 25 °C: solid
Melting point: 115.21 °C
Boiling point: 444.6 °C
Density: 2.067 g/cm³
Electronegativity (Pauling): 2.58
Discovered: Prehistoric times, as brimstone

Neighbours in the table

P Cl

Sulfur at room temperature is a solid of rings, not loose atoms. Each ring holds eight atoms in a puckered loop that OpenStax likens to a crown, so the element’s true formula is S₈. Behind that shape sit sixteen electrons, 2, 8 and 6, written [Ne] 3s² 3p⁴, six of them outer as in oxygen. Nonmetal atoms usually bond until eight electrons surround them; group 16 atoms, two short, need two covalent bonds.

Oxygen spends both on one partner, the double bond of O₂. Sulfur does not normally form a stable pi bond to itself, the second link of a double bond. OpenStax puts that down to size: a group’s small first member overlaps its orbitals better. So each sulfur atom makes two single bonds, one per neighbour, and the neighbours do the same until the chain closes into a ring.

The everyday form is yellow rhombic sulfur, named for its crystal shape. Sulfur melts at 115.21 °C to a runny liquid, because the S₈ rings are nearly spherical and slip past one another easily. Heated further, the rings break into chains that join end to end and tangle, until at about 230 °C the liquid hardly pours. It turns dark red too: atoms dangling at chain ends absorb light differently from atoms bonded on both sides.

Where it is found

PubChem’s record gives 350 milligrams of sulfur per kilogram of crust and 905 per litre of sea water. Jefferson Lab’s text there ranks it tenth in the universe by abundance. OpenStax adds a newer sighting, the atmospheres of Venus and of Jupiter’s moon Io.

The free element collects near volcanoes and hot springs, deposits that may come from hydrogen sulfide in volcanic gases being oxidised. Far more is locked in minerals: iron in pyrite, lead in galena, zinc in sphalerite, calcium sulfate in gypsum and barium sulfate in barite.

The Frasch process mines deposits under Texas and Louisiana. Water at 170 °C and 10 atm goes down the outermost of three concentric pipes and melts the sulfur, whose melting point is 115.21 °C. Compressed air goes down the innermost pipe, and the melt brought up separates on cooling, 99.5 to 99.9 per cent pure.

Most sulfur now comes from processing fossil fuels: hydrogen sulfide stripped from natural gas is oxidised to sulfur and water. The Geological Survey puts the 8.2 million tons the United States recovered in 2024 to refineries, gas-processing plants and coking plants, in that order.

Why it matters

About nine tenths of the sulfur the United States consumed in 2024 went into sulfuric acid. OpenStax notes that industry uses more of it than of any other manufactured compound. The route runs through burning: oxygen, the more electronegative of the two (the stronger pull on the electrons of a bond it shares), oxidises sulfur to sulfur dioxide. Heating the dioxide with more oxygen slowly gives sulfur trioxide, which is converted to the acid. Phosphate rock treated with the concentrated acid yields phosphoric acid; fertiliser, lead-acid batteries and nickel for battery materials all draw on the acid.

The element was used long before anyone knew it was one. Jefferson Lab’s text in PubChem’s record has Homer naming sulfur as a pest deterrent nearly 2,800 years ago. Los Alamos still lists a fungicide among its uses, beside vulcanising rubber and bleaching dried fruit. Jefferson Lab says Antoine Lavoisier convinced the scientific community around 1777 that sulfur was an element; IUPAC’s text credits Gay-Lussac and Thenard with proving it in 1809.

Many proteins contain sulfur, and its four stable isotopes leave a signature in living tissue. The ratio of sulfur-34 to sulfur-32 in an animal’s tissue tells whether it grew in fresh water or the sea. In a bird’s feathers the same ratio records where the bird lived while they grew.

Properties

Properties of sulfur Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	16
Standard atomic weight	32.06(2) Da	Da	IUPAC/CIAAW 2024
Melting point	115.21 °C	°C	Los Alamos: 115.21 °C
Boiling point	444.6 °C	°C	Los Alamos: 444.6 °C
Density	2.067 g/cm³	g/cm³
Electronegativity (Pauling)	2.58
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁴
Phase at 25 °C	solid

The standard atomic weight of this element is an interval in nature, from 32.059 to 32.076 Da; the value shown is IUPAC's conventional abridged value.

Safety

Elemental sulfur is a low-hazard solid rather than a harmless one. The EU classification is a single statement, skin irritation, under the signal word Warning; 99.7 per cent of 2,904 company reports to the European Chemicals Agency agree. PubChem’s safety summary adds that the solid is combustible, burning to sulfur dioxide, that its dust suspended in air ignites easily, and that molten sulfur causes severe burns. Those cautions concern people who handle the powder or the melt, in industry or on farms, and the Laboratory Chemical Safety Summary is the sheet to read.

Questions

Does sulfur smell like rotten eggs?

No. Los Alamos describes the element as a pale yellow, odourless solid, and its safety sheet notes that evaporation at 20 °C is negligible. The rotten-egg smell belongs to hydrogen sulfide, a compound of sulfur and hydrogen that also gives many hot springs their odour. The smell of burning sulfur is different again: OpenStax traces it to sulfur dioxide, the gas the element forms as it burns.

Why is sulfur called brimstone?

Brimstone comes from brenne stone, meaning combustible stone, according to the IUPAC text in PubChem's element record; the Latin sulphurium and the Sanskrit sulveri stand behind the modern name. The older word describes what the yellow solid does when lit: PubChem's safety data record a blue flame, difficult to see in daylight, that produces sulfur dioxide. Los Alamos notes that Genesis calls the element by the older name, and OpenStax identifies the brimstone of the Bible with sulfur.

Do we need sulfur in our diet?

Yes. The nutrition table in OpenStax's Anatomy and Physiology lists sulfur among the trace minerals as a component of amino acids, with eggs, meat, poultry, fish and legumes as its sources; minerals cannot be made in the body and must come from food. A shortfall shows up as protein deficiency. Los Alamos adds that fats, body fluids and the minerals of the skeleton all hold a little sulfur.

Where does the world's sulfur come from?

Mostly from processing fossil fuels, the US Geological Survey says, which is why it expects supplies to stay adequate for the foreseeable future. World production in 2024 was about 85 million tons; China made 19 million and the United States 8.2 million. Oil and sulfide ores can be processed far from where they were dug, so the producing country is not always the one holding the reserves. Canada supplied 70 per cent of US sulfur imports in 2020–23.

Is sulfur a metal or a nonmetal?

A nonmetal: Los Alamos calls the solid a good insulator, and OpenStax explains that nonmetals form no single-atom positive ions because the energy needed to remove an electron from them is too high. Sulfur's own ion is the sulfide anion, charge 2−, made by oxidising metals. Oxygen and the halogens, more electronegative still, oxidise sulfur instead: excess fluorine takes it to one sulfur bonded to six fluorines, while iodine does not react with it.

Compounds containing sulfur

H₂S Hydrogen sulfide 3 atoms, 2 single bonds.
SO₂ Sulfur dioxide 3 atoms, 2 double bonds.
SO₃ Sulfur trioxide 4 atoms, 3 double bonds.
BaSO₄ Barium sulfate 6 atoms in one formula unit of an ionic lattice.
CaSO₄ Calcium sulfate 6 atoms in one formula unit of an ionic lattice.
CuSO₄ Copper(II) sulfate 6 atoms in one formula unit of an ionic lattice.
MgSO₄ Magnesium sulfate 6 atoms in one formula unit of an ionic lattice.
H₂SO₄ Sulfuric acid 7 atoms, 6 bonds, tetrahedral.

All 11 compounds containing sulfur

C₃H₇NO₂S Cysteine 14 atoms, 13 bonds.
C₁₆H₁₈N₂O₄S Penicillin G 41 atoms, 43 bonds.
C₁₆H₁₉N₃O₅S Amoxicillin 44 atoms, 46 bonds.