why are halogen so reactive?

Hi Student
The Group 7 elements are also known as the halogens. They include fluorine, chlorine, bromine and iodine, which all have seven electrons in their outer shell i.e They only need one more electron, making eight, to have stability, so they try their absolute hardest to get it.
This high reactivity is due to the high electronegativity of the atoms due to their high effective nuclear charge. They can gain an electron by reacting with atoms of other elements.

Atoms of elements consist of a nucleus that contains protons and neutrons. The nucleus is surrounded by electrons that move around in orbits or levels. Atoms of an element react with other atoms based on the number of electrons found in their outer orbit. These outer electrons are called valence electrons. For an atom of an element to be nonreactive it must have eight electrons in its outer level. All members of the halogen family have seven valence electrons.

Because these atoms are so close to having a full set of eight valence electrons, they're very reactive. They show a chemical tendency to gain one electron by forming bonds and taking the electron from other atoms to attain a full set. Because of their chemical reactivity, these elements do not exist as single elements in nature. They are only found in compounds.