Question

Why are halogens so reactive?

Answer 1

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  

Answer 2

This means that has great capacity to react with other chemical elements in nature, reacting mainly with sodium, therefore, can only be findings in chemical compounds in most cases. 

hope this helps!.