Halogens reactivity decreases down the group, when halogens react it gains an electron to form a singly negative charged ion.
Halogens are reactive because they only need one additional electron to complete their octet. As it go down the group from Fl to Cl to Br to I the size of the atom increases due to an extra filled electron shell. The valence electrons are further and further away from the nucleus, the attraction of forces between the electrons and the nucleus become weaker and weaker