Halogens are extremely reactive elements. Great amounts of halogen can be very harmful or lethal to any biological organisms. Halogens are often called ‘oxidizers’ due to their high reactivity towards any element. What causes their high reactivity is their composition. Halogens only have seven valence electrons, unlike noble gasses which have eight valence electrons. These halogens react with the atoms of other elements in order to gain or lose extra electrons to become like the nearest noble gas. They can do this through sharing electrons or in the case of covalent compounds. Halogens will do anything to gain or lose anything just to become like the nearest noble gas.
Why do they want to become like the nearest noble gas? The answer is because they want to have stable structures like the noble gasses due to their filled octet.
Aside from their valence count, their atoms also play a great role in their reactivity. Halogen atoms have a high effective nuclear charge that makes them highly electronegative which in turn causes the halogens to become highly reactive elements. They react to metals and to nearly all non-metals except oxygen, neon, nitrogen and helium.
One of the most reactive halogen elements is the fluorine. It is a highly corrosive and highly toxic gas. They form compounds with the heavier noble gasses. Fluorine is highly reactive because once it reacts with an element, it bonds with it so strongly making the resulting molecules very inert and non-reactive to anything else.
Bromine and chlorine are common elements which are used as disinfectants for fresh wounds, spas, swimming pool and drinking water. Through the process called sterilization, they kill various bacteria and other harmful microorganisms present in the water. Chlorine is also used in most fabric bleaches. The reactivity of each halogen element decreases from fluorine to iodine.
In conclusion, halogens are very reactive because of their composition and structure. They will do anything to gain or lose any electron to become a noble gas.