As the size of the halogen atom increases from F to I, H – X bond length in HX molecules also increases from H – F to H – I (H – F < H – CI < H – Br < H – I). The increase in H – X bond length decreases the strength of H – X bond from H – F to H – I. The decrease in the strength of H – X bond is evident from the fact that H – X bond dissociation energies decrease from H – F to H – I . Due to successive decrease in the strength of H – X bond from H – F to H – I, thermal stability of HX molecules also decreases from HF to HI (HF > HCI > HBr > HI).