Here's why:

* Valency refers to the combining power of an atom or ion. It's essentially the number of electrons an atom can gain, lose, or share to form a chemical bond.

* Thiosulfate is an anion (negatively charged ion) with a charge of -2.

* The -2 charge indicates that the thiosulfate ion has gained two electrons.

Therefore, the valency of thiosulfate is 2, meaning it can form bonds with two other atoms or ions to achieve a stable configuration.