The halogens form covalent bonds with each other. They exist in the elemental form as diatomic molecules (F₂, Cl₂, Br₂, I₂)

When halogens react they mostly form the halide ions with oxidation state of -1 (F^-, Cl^-, Br^-, I^-)
The reactivity of the halides depends on their affinity for electrons. It is obvious that fluorine is the most reactive halide and reactivity decreases as we go down the group. The affinity for electrons decreases as the valence electrons move further from the nucleus as we progress down the group.

Hydrogen halides dissolve in water to give acidic solutions according to the reaction below.(X represents a halide atom)

HX_(g) + H₂O_(l) => X^-_(aq) + H₃O⁺_(aq)

The electronic configuration of these elements is as follows.
F 1s², 2s² 2p⁵

Cl 1s², 2s², 2p⁶, 3s², 3p⁵

Br 1s², 2s², 2p⁶, 3s², 3p⁶, 4s², 4p⁵

I 1s², 2s², 2p⁶, 3s², 3p⁶, 3d¹⁰, 4s², 4p⁶, 5s², 5p⁵

Notice how there is always 7 outer shell electron. All elements in group 7 have similar properties due to the similarity in their electronic configuration.