The halogens are a group of five non-metallic elements, including fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). The reactivity of halogens decreases down the group. This means that fluorine is the most reactive and astatine is the least reactive.

The reactivity of halogens is due to their ability to gain an electron to form a negatively charged ion (anion) with a noble gas configuration. The reactivity increases as the distance between the nucleus and the outermost electrons decreases. As we go down the group, the number of energy levels or shells increases, and the outermost electrons are farther from the nucleus, which results in a decrease in reactivity.

The following are some examples of the reactivity of halogens:

• Fluorine is the most reactive halogen, and it reacts violently with water and other substances. It can even react with the glass.
• Chlorine is less reactive than fluorine, but it is still quite reactive. It reacts with water to form hydrochloric acid and oxygen gas.
• Bromine is less reactive than chlorine, and it is a liquid at room temperature. It reacts with water to form hydrobromic acid and oxygen gas.
• Iodine is even less reactive than bromine, and it is a solid at room temperature. It reacts with water to form hydriodic acid and oxygen gas.
• Astatine is the least reactive halogen and is rarely found in nature. It is a radioactive element, and its properties are not well understood.

In summary, the reactivity of halogens decreases down the group, with fluorine being the most reactive and astatine being the least reactive.

HI＞HBr＞HCl＞HF 

HI is a very strong acid because HI easily breaks up and form H+ Ion in water due to weak Covalent Bonding.

HF is a very weak acid because it has strong Covalent Bonding so it does not break up easily to form H+ Ions and reacts with a water molecule to form Hydronium(H3O+).