The halogens form compounds with hydrogen called hydrogen halides. Hydrogen chloride is HCl, hydrogen bromide is HBr and hydrogen iodide is HI. Hydrogen reacts with chlorine to make hydrogen chloride: H2 + Cl2 → 2HCl. The hydrogen halides are molecular compounds and they are gases at room temperature.
The hydrogen halides are very soluble and dissolve in water to form acidic solutions. In water the molecules form hydrogen ions, H+, and halide ions, so the solution is acidic and has a pH below 7. Hydrogen chloride dissolves to make hydrochloric acid.
All the halogens follow the same pattern. Fluorine should form hydrogen fluoride, HF, and astatine should form hydrogen astatide, HAt, and both should dissolve in water to give acidic solutions.