Sulfur dioxide

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3 atoms, 2 double bonds.

IUPAC name: sulfur dioxide
Molar mass: 64.058(22) g/mol
Atoms: 3
SMILES: O=S=O
InChIKey: RAHZWNYVWXNFOC-UHFFFAOYSA-N
PubChem CID: 1119

Made of

Structural formula of sulfur dioxide. Every atom in the molecule is drawn. It shows which atoms are joined to which, not the shape. The model above shows the shape.

Power plants and other industrial facilities burning fossil fuels are the largest source of sulfur dioxide in the atmosphere, according to the US Environmental Protection Agency (EPA). Each molecule holds one sulfur atom and two oxygen atoms, and its molar mass is 64.06 g/mol. The CAMEO Chemicals database describes its odour as choking or suffocating.

The gas weighs 2.619 g/L, heavier than air, and vapour from the liquefied product at first creeps along the ground. It boils at about −10 °C and liquefies readily under compression; industry ships it as a liquid in pressurised cylinders. It does not burn, and HSDB, the Hazardous Substances Data Bank, reports that it stays stable when heated as far as 2000 °C.

In water the gas dissolves very easily and reacts to form sulfurous acid, H₂SO₃. That puts it in the family of nonmetal oxides which, in Chem1’s account, tend to give acidic solutions when added to water.

Structure and bonding

The viewer holds three atoms: a yellow sulfur atom in the middle and a red oxygen atom on each side. Each sulfur–oxygen link is drawn as a double bond, the structure PubChem writes as O=S=O. In PubChem’s computed model the two bonds are the same length, 1.570 Å, and they meet at an angle of 113.0° rather than lying in a straight line.

Ball-and-stick shows that angle most clearly, with the two oxygen spheres both swung to one side of the sulfur. Stick drops the spheres and leaves only the two bonds, and the angle at sulfur becomes the whole picture. Space-filling swells each atom to its full size: the sulfur, the larger sphere, sits partly buried between two smaller oxygen spheres. The Labels button prints S and O on the atoms.

The drawing is the free SO₂ molecule of the gas. Dissolved in water, it can react to form sulfurous acid, H₂SO₃, according to NIOSH, the US National Institute for Occupational Safety and Health.

Where it turns up

Most sulfur dioxide in the air comes from burning coal and oil in power stations or from smelting copper, according to ATSDR, the Agency for Toxic Substances and Disease Registry. Extracting metal from ore also releases it, as do locomotives, ships and heavy equipment burning fuel high in sulfur, according to the EPA. Volcanic eruptions release it naturally.

Workers meet it in plants that make sulfuric acid, paper, food preservatives and fertilisers. In food and drink, HSDB lists its use to preserve fruit and to make molasses, wine and beer. Grapes are treated with it after harvest as a fungicide and preservative. CAMEO records it as a disinfectant in breweries and canneries.

Homes see it too, where biomass fuel is burned for cooking and heating, according to Haz-Map. Metabisulfite, a preservative, can release toxic amounts of the gas when it meets water and acid.

Why it matters

HSDB names sulfur dioxide as an intermediate in making sulfuric acid, and as a raw material for sulfites, hydrosulfites and thiosulfates. It bleaches wood pulp and paper, treats waste and water, and serves in metal, ore and oil refining. CAMEO calls it a reducing agent, though not a powerful one, and a reducing bleach that decolourises many materials. In food, HSDB lists it for holding back browning, enzyme-driven reactions and bacterial growth.

The same gas pollutes the air. The EPA treats it as the indicator for all gaseous sulfur oxides because it is the one of greatest concern. Cutting SO₂ can generally be expected to cut exposure to the whole group. In air it changes into sulfur trioxide, sulfates and sulfuric acid, according to ATSDR. The EPA says sulfur dioxide and related sulfur oxides can add to acid rain, which harms sensitive ecosystems. The particles it forms blur the view in national parks and stain statues and monuments.

Properties

Properties of sulfur dioxide Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 1119.
Property	Value	Unit	Source
Molecular formula (Hill order)	O₂S
Molar mass	64.058(22) g/mol	g/mol	IUPAC/CIAAW 2024
Sulfur atoms	1
Oxygen atoms	2
Bonds	2
PubChem molecular weight	64.07 g/mol	g/mol	PubChem

Safety

Sulfur dioxide is a toxic, corrosive gas, and outdoors people meet it as air pollution. Most notifications to the European Chemicals Agency (ECHA) classify it as toxic if inhaled and as causing severe skin burns and eye damage. People with asthma, children especially, are sensitive to its effects, the EPA notes, and the liquid causes frostbite, according to NIOSH. Its hazards are set out in full in the PubChem Laboratory Chemical Safety Summary, and its workplace exposure limits on the NIOSH Pocket Guide card.

Questions

Why does sulfur dioxide mostly affect the nose and throat?

Sulfur dioxide dissolves very readily in water, and therefore, HSDB reports, it is absorbed efficiently in the upper airways. The nose filters out most of the inhaled gas, keeping it from sensitive irritant receptors in and below the larynx, the voice box. Breathing through the mouth, by the same account, substantially increases the share of sulfur dioxide that reaches the lungs.

What does sulfur dioxide smell like?

CAMEO describes the odour as choking or suffocating, and the NIOSH card calls it pungent and irritating; HSDB records an acid taste. The irritation works as an alarm. Because the gas dissolves so readily in water, it stings the eyes and upper airway hard, according to HSDB, and that warns an exposed person to escape before serious damage occurs.

How does sulfur dioxide cause acid rain?

Once in the air, the gas can turn into sulfuric acid, sulfur trioxide or sulfates, according to ATSDR. The EPA says sulfur oxides contribute to acid rain that can harm sensitive ecosystems. Like other nonmetal oxides, it gives an acidic solution in water, forming sulfurous acid, H₂SO₃, as Chem1 shows. At high concentrations, the EPA says, the gaseous oxides can also damage the leaves of trees and plants and slow their growth.

Is sulfur dioxide made in the human body?

Yes. HSDB reports that mammals make it from the amino acid L-cysteine. The enzyme cysteine dioxygenase turns L-cysteine into L-cysteine sulfinate, and aspartate aminotransferase turns that into beta-sulfinylpyruvate. This breaks down on its own to pyruvate and SO₂. Studies in animals point to effects on blood vessels and the heart. The enzyme sulfite oxidase appears to convert the gas rapidly to sulfate, which leaves mainly in urine.

Made of the same elements

SO₃ Sulfur trioxide 4 atoms, 3 double bonds.