Answer:
They have seven electrons in their valence shell, so halogens are very reactive.
Step-by-step explanation:
The halogens are the elements of the group 17 (or 7A) of the periodic table, all the elements have the valence shell
, so they have 7 electrons in their valence shell.
These elements need only one electron to be stable according to the octet rule, so they are very reactive, they can react with metals, non-metals, hydrogen and with themselves.