When group 7 elements react , their atoms gains one electron into their highest occupied energy level ( outer shell) to form single negative - charged ion .
As you move down the group , the outer shells get increased i.e an additional electron shell is added thereby increasing the atomic radius of the atom. The electrons in the outer shell move further away from the nucleus as we go down the group .
Therefore the force of attraction between the shells and nucleus is decreased .
This makes attracting an extra electron difficult . And so ability of the atom to attract electron to fill the outermost shell reduces which means the reactiveness of the atom reduces.
