The correct order of the thermal stability of hydrogen halides (H-X) is

he thermal stability of hydrogen halides (H-X) generally follows the order:

HF > HCl > HBr > HI

This is because the strength of the hydrogen-halogen bond decreases down the group, and the bond strength is directly related to the thermal stability of the hydrogen halide. HF has the strongest hydrogen-fluorine bond and is therefore the most thermally stable, while HI has the weakest hydrogen-iodine bond and is the least thermally stable.

Additionally, the bond polarity and the size of the halogen atoms also influence the thermal stability of hydrogen halides. HF has a very high bond polarity due to the large electronegativity difference between hydrogen and fluorine, which increases its stability. In contrast, the larger halogens (Cl, Br, I) have weaker bond polarity and their larger size also leads to weaker bond strength and lower thermal stability.