Halogens include group 17. They form diatomic molecules, (two of the same atoms bonded together.)
They have large electron affinities. Reactivity decreases down the column. Fluorine differs from other halogens in that it cannot expand its valence shell, and almost always forms one bond. It also has a tendency to form ionic bonds with metals. Fluorine stabilizes other elements in higher oxidation states.
Physical form at room temp:
Electronegativity:(you may find other values depending on the method used to derive the EN values and whether the standard is set at 4).
When the halogens are hydrated they for hydrohalic acids which, with an exception of HF, form the strong acids. To prepare HF and HCl, a halide salt heated in the presence of a nonvolatile acid such as sulfuric acid will produce the HF or HCl acid in the gas phase.
CaF2 (s) + H2SO4 (con. aq) → 2 HF (g) + CaSO4 (aq)
Because sulfuric acid is an oxidizing agent, HBr and HI cannot be produced this way. In the presence of sulfuric acid they are reduced to Br2 gas and I2 gas. Instead, H3PO4 acid must be used which is nonvolatile and nonoxidizing.
All of these acids can be produced by directly mixing the elements needed together such as H and F, but this usually is very violently explosive.