Thermal stability is maximum in ____ ?

Hydrogen reacts with halogens to give hydrogen halides but affinity for hydrogen decreases from fluorine to iodine. The stability of these halides decreases down the group due to decrease in bond (H–X) dissociation enthalpy in the order: H–F > H–Cl > H–Br > H–I.

High bond dissociation enthalpy in HF indicates that HF has the maximum thermal stability.