7. What happens to the reactivity of the halogens as the group is descended?

Question

i thought nothing as they have a full outer shell. but then the question wouldn't make sense.

"(explain your answer with reference to the atomic structure, write a balanced symbol equation)"

Answer 1

The halogens are one electron short of having a Noble Gas structure, so they readily gain one electron to become X- ions.

i.e.

X2(g) + 2e --> 2X-(g)

As the Group is descended the atoms increase in size and so attract an incoming electron less strongly: so they become less reactive as the Group is descended.

Answer 2

Halogens are the elements in group 7A or 17 of the periodic table.  Their "outer shells" called valence shells are not "full".  In fact, they have seven valence electrons, and are only one electron away from the perfect octet, just like noble gases.  Therefore they have the tendency to grab one extra electron wherever they can find it.  This tendency to attract electrons is known as electronegativity.  It depends on the attraction that the positive nucleus has on negative electrons.  This attraction depends in part on the distance between valence electrons and the nucleus of the atom, and on the number of valence electrons already present in the atom.  Since halogens already have a lot of those valence electrons (7), the ones with fewer electron shells attract extra electrons more than the ones with a greater number of shells.  Therefore, fluorine that only has two shells and is at the top of the group is most reactive because it has the greatest attraction for electrons.  Chlorine has three shells, and it is slightly less reactive.  Then comes bromine with four shells, and still lesser reactivity, followed by iodine, the least reactive of halogens.

I am not sure what you mean by "write a balanced symbol equation".