Question

Why does `HI` have a higher boiling point than `HBr`, and `HCl`?

Answer 1

The hydrogen iodide molecule is larger and more polarizable than hydrogen chloride..............

Explanation:

Given its polarizability, the diffuse electron cloud of the `HI` molecule allows more effective (i.e. more attractive) intermolecular interaction. Given this enhanced intermolecular interaction, `HI` should have a higher `"boiling point/melting point"` than `HCl`. You will have to look up the physical constants, [here is a start.](https://www.quora.com/Out-of-HF-HCl-HBr-and-HI-which-has-the-lowest-and-highest-boiling-point-and-why)

But hang on, `HF`, has a boiling point of `19.5` `""^@C`, which is the highest of the hydrogen halides. Doesn't this contradict what I have said? What's going on?