Science is easy. All you have to do is break the words down.
Fluoride - flu - now we know that is bad. Ride - something you do when you can't walk so that is bad, too. Conclusion, fluoride is bad.
Chloride - chlo, as in chlorophyl, the stuff that plants have that makes our breath smell good. Ride, same as above. Chloride is a wash.
Iodine - I, me, myself, and I. Good. Dine, lots of good groceries. Iodine is very good.
Halogen - halo, the thing around the heads of religious icons. Got to be good. Gens for generations or gins, both good. Halogens are good.
Once you get the hang of breaking down the words these things are a snap. Go in peace and be a happy scientist.
Nice one. I have never heard that before.