ChemSphere
16S32.06

Sulfur

Reactive nonmetal · Period 3 · Group 16 · p-block

Brimstone of volcanoes, backbone of industry

Atomic mass
32.06 u
State at 25 °C
Solid
Electronegativity
2.58
Melting point
115.2 °C
Boiling point
444.7 °C
Configuration
[Ne] 3s2 3p4
Building atom…
16 protons 16 neutrons 16 electrons

Sulfur, explained

Overview

Sulfur sits below oxygen in group 16 with six valence electrons ([Ne]3s2 3p4), but its larger atom is less electronegative (2.58) and much happier bonding to itself. Instead of forming S2 molecules like O2, sulfur atoms link into crown-shaped rings of eight, S8, which pack into the familiar yellow crystals. Melted at 115 °C, sulfur is a runny yellow liquid; heated past about 160 °C, the rings break open and join into long chains, and the liquid turns dark red and thick as syrup. Sulfur's oxidation states range from -2 in sulfides to +6 in sulfates.

Sulfur's most important compound is sulfuric acid, made in larger quantities than any other industrial chemical, more than 250 million tonnes a year, mostly for phosphate fertilizers. Today nearly all sulfur is recovered by removing hydrogen sulfide from natural gas and crude oil, rather than mined. Volcanoes release sulfur dioxide, which can cool the climate, while SO2 from burning coal causes acid rain. In biology, sulfur is part of the amino acids cysteine and methionine, and disulfide bridges between cysteines help proteins hold their shape.

Discovery

Sulfur has been known since antiquity as brimstone, 'burning stone', and was used to fumigate homes, bleach cloth and, from about the 9th century in China, make gunpowder. Its status as an element was settled by Antoine Lavoisier, who argued in 1777 that it was an element rather than a compound. The name comes from the Latin sulpur; in 1990 IUPAC adopted the spelling 'sulfur' as the international standard.

Where it’s found
Native sulfur occurs near volcanoes and in salt domes, but most sulfur is now recovered from sour natural gas and petroleum by the Claus process.
What it’s used for
  • Sulfuric acid production
  • Sulfate fertilizers
  • Rubber vulcanization
  • Black powder
  • Agricultural fungicides
  • Lead-acid batteries
In living things
Sulfur is essential: it is found in the amino acids cysteine and methionine, in iron-sulfur clusters of enzymes and in vitamins such as biotin and thiamine. Hydrogen sulfide and sulfur dioxide are toxic gases.
Safety
Elemental sulfur has low toxicity, but its dust is flammable and burning sulfur releases choking, toxic sulfur dioxide. Hydrogen sulfide is deadly at high concentrations and quickly paralyses the sense of smell, so its odour cannot be relied on as a warning.

How it compares

Where Sulfur sits among all 118 elements on the trends that shape the table.

  • Electronegativity2.58Pauling

    #9 of 100 elements with data

  • Ionization energy1,000kJ/mol

    #17 of 104 elements with data

  • Atomic radius100pm

    #85 of 95 elements with data

  • Electron affinity200kJ/mol

    #8 of 109 elements with data

  • Density2.07g/cm³

    #92 of 114 elements with data

  • Melting point388K

    #87 of 110 elements with data

Density
2.07 g/cm³
Atomic radius
100 pm
Covalent radius
103 pm
Molar heat
22.75 J/(mol·K)
Appearance
lemon yellow sintered microcrystals
Discovered by
Ancient china

Inside the atom

16 electrons arranged in 3 shells: 2 · 8 · 6.

Orbital diagram

1s2 2s2 2p6 3s2 3p4 · shorthand [Ne] 3s2 3p4

  1. 1s2
  2. 2s2
  3. 2p6
  4. 3s2
  5. 3p4

Each box is an orbital holding up to two electrons of opposite spin. Highlighted subshells hold the valence electrons that decide how Sulfur bonds.

Ionization energies

Energy to remove each electron in turn, on a log scale. The biggest leap, from electron 14 to 15 (×4.6), is where the next electron has to come from a filled shell much closer to the nucleus.

Signature reactions

The chemistry Sulfur is known for.

  1. Equilibrium · Industrial

    Contact process: oxidizing sulfur dioxide

    Sulfur dioxide is oxidized to sulfur trioxide over a vanadium oxide catalyst, the key step in making sulfuric acid. The reaction is exothermic, so a moderate temperature is a compromise between yield and speed. The SO3 is absorbed into concentrated sulfuric acid and then diluted.

    2SO2(g)+O2(g)2SO3(g)

    Conditions: vanadium(V) oxide catalyst, about 450 °C

  2. Redox · Comproportionation

    Claus process

    Hydrogen sulfide removed from natural gas reacts with sulfur dioxide (made by burning part of the H2S) to give elemental sulfur and water. Sulfur in the -2 and +4 oxidation states meets in the middle at 0. Refineries and gas plants worldwide use this process, producing most of today's sulfur.

    2H2S(g)+SO2(g)3S(s)+2H2O(g)

    Conditions: alumina catalyst, about 200-350 °C

  3. Redox · Combustion

    Gunpowder combustion (simplified)

    In black powder, potassium nitrate supplies oxygen, charcoal is the fuel and sulfur lowers the ignition temperature and speeds the burn. The sudden release of hot gases propels bullets and fireworks. The real reaction produces a messier mixture of products, but this equation captures the essentials.

    2KNO3(s)+S(s)+3C(s)K2S(s)+N2(g)+3CO2(g)

    Conditions: ignition

Watch Sulfur react in 3D

Where you’ll meet it

Research

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