Why do halogens form diatomic molecules?

Halogens are inherently almost inert gases that lack one electron to have a fully filled electronic level and become similar to “noble gases”. Thus, halogens form binary compounds with elements that can donate one electron: NaI, KF, HBr, LiCl. But halogens do not always form only diatomic molecules; in compounds with a stronger oxidizing agent, halogens donate electrons and can form triatomic molecules – HClO.