Halogens are reactive because:
They have high electronegativity and also high nuclear charge. So, they are reactive and also gain an electron when they react with other elements.
Since they are very reactive, halogens are very harmful to living organisms. Some of the halogens are fluorine, chlorine, bromine, iodine, astatine. These are mostly non metals. Fluorine is one of the most reactive gas and also very toxic gas. When Fluorine reacts with glass along with small amounts of water, it forms silicon tetra fluoride (SiF4). Hence fluorine should be handled with substances like the inert organofluorine compound Teflon.