 Hydrogen halide

A hydrogen halide has formula $\mathrm{HX}$, with hydrogen bonded to a halogen. Hydrogen fluoride, hydrogen chloride, hydrogen bromide, and hydrogen iodide are familiar examples.

Their gas-phase molecules must be distinguished from their aqueous acid solutions. Bond strengths, solvation, and association change across the series, so increasing bond polarity alone does not predict every acid-strength trend.