Which one of the following is most reactive? 1.Cl2 2.Br2 3.I2 4.ICl

##”ICl”##
I would have to say iodine monochloride, ##”ICl”##, which is a diatomic interhalogen compound.
The reactivity of diatomic halogens actually decreases as you move down group 17 of the periodict. ##”F”_2## is by far the most reactive of the group, but as you go down to ##”Cl”_2##, ##”Br”_2##, and ##”I”_2##, reactivity decreases.
This happens because as you move down the group, the size of the atoms, and implicitly of the molecules, increases.
This means that the molecules have bigger electron clouds and will experience stronger London Dispersion Forces as you move down the group.
On the other hand, iodine monochloride is very reactive because of the difference in between iodine and chlorine, which makes for a .
The molecule will actually release ##”I”^(+)## cations in solution.