The S
3 molecule or trisulfur or sulfur trimer or thiozone or triatomic sulfur is an allotrope of sulfur. It occurs as a mixture in liquid and gaseous sulfur and also at cryogenic temperatures as a solid. Under standard conditions it is unstable and self reacts to solid sulfur cyclooctasulfur. The molecule shape is similar to ozone.S
3 is found in sulfur vapour, comprising 10% of vapour species at 713 K (440 °C; 824 °F) and 1,333 Pa (10.00 mmHg; 0.1933 psi). It is cherry red in colour, with a bent structure, similar to ozone, O
3. The bonds between the atoms are not full double bonds (as this would require two fewer electrons, similar to carbon disulfide), and the molecule can be thought of as a resonance between two states, in each of which one of the end atoms has a negative formal charge while the central atom has a positive formal charge.

The molecule has a distance between sulfur atoms of 191.70 ± .01 pm (1.9170 ± 0.0001 Å) and angle at the central atom of 7000204831841014054♠117.36°±0.006°. However, cyclic S
3, where the sulfur atoms are arranged in an equilateral triangle with three bonds (similar to cyclic ozone), should in theory be lower in energy than the bent structure actually observed.

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