Halogen belongs to VII-A group and consists of Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I) and Astatine (At). Halogens exist in Molecular form. The reactivity of halogens decreases down the group because atomic size increases, and electro negativity decreases down the group.
Halogens act as oxidizing agent, because they easily accept electron.
\ce{2Na + Cl2 → 2NaCl}
\ce{2K + l2 → 2Kl}
Power of Halogens as oxidizing agent decreases down the group it means Fluorine can displace other Halogens due to its highest oxidizing power.
Reaction of Hydrogen with Halogen form Halogen acid. The Acidic strength of Halogen Acid decreases in the following order:
HI > HBr > HCl > HF
HI is very strong acid because HI easily breaks up and forms H Ion in water due to weak Covalent Bonding. HF is very weak acid because it has strong Covalent Bonding so it does not break up easily to form \ce{H+} Ion in water. H Ions reacts with water molecule to forms Hydronium (\ce{H3O+} ) Ion.
