S₂O₃ is the chemical formula for thiosulfate. It's an anion with a charge of -2, often found in salts like sodium thiosulfate (Na₂S₂O₃), which is commonly used in photography.

Here's a breakdown:

* S: Sulfur

* O: Oxygen

* 2: Two sulfur atoms

* 3: Three oxygen atoms

Thiosulfate is structurally similar to sulfate (SO₄²⁻), but with one oxygen atom replaced by a sulfur atom.