Benzene reacts with Chlorine gas in the presence of iron trichloride to yield hexachlorobenzene. However, when it reacts with fluorine gas, it forms a quinoid product (I would actually draw it for them - no need to know the name). Why the difference?
Fluorine is much more reactive than chlorine, even destroying the aromaticity. This is at the expense of the very strong C-F bonds (good orbital size and energy overlap) that are formed. C-Cl bonds are weaker so even when benzene is "burnt" in chlorine, the aromatic ring stays intact.
RB
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