11.2.3—Thermal stabilities of the hydrogen halides

Thermal stability describes resistance to decomposition on heating. Hydrogen halides become less thermally stable down Group 17 as the H–X bond lengthens and weakens.

A larger halogen atom gives poorer orbital overlap with hydrogen, so less energy is needed to break the bond. This bond-strength explanation is distinct from the aqueous acid-strength question, even though both involve H–X cleavage.

HI decomposes more readily than HCl when heated. A comparison should mention bond length/overlap, not merely say iodine is “more reactive”.

Thermal stability is not the same as boiling point, volatility or acid strength; identify the process being measured.