# Why is HF considered a covalently bonded compound?

Why is the bond between H and F in HF considered polar covalent whereas HCl, HI, and so on are all ionic? The electronegativity difference between them suggests that it too should be ionic, yet all textbooks say that HF is covalent. Is there a good reason why?

• @EricBrown Indeed, that's an interesting fundamental reason why an ionic salt containing discrete non-solvated $\ce{H^+}$ ions is all but impossible; the free hydrogen cation has an immense ability to polarize electron clouds (which I have discussed elsewhere), demanding a high degree of covalent bonding with even the most unwilling species. However, I guess that argument does not strictly preclude an ionic $\ce{HX}$ compound with, for example, discrete $\ce{HX2^{-}}$ and $\ce{H2X^{+}}$ ions, at least qualitatively. – Nicolau Saker Neto Jun 1 '14 at 14:33
• @EricBrown See, e.g., bifluoride and fluoronium, for examples of $\ce{HX2-}$ and $\ce{H2X+}$ species (more links to related content therein). – hBy2Py Jul 30 '15 at 19:51