I like your question about fluoride and other halogens in the teeth. Fluoride is so toxic that we aren't supposed to swallow our toothpaste. Fluoride is the smallest halogen ion (= 'halide'). Chloride, bromide, and iodide, are increasingly larger. Astatine (At) is the largest halogen and is very radioactive. Chloride - we need this in our bodies in large amounts. Bromide - it was used as a sedative 100 years ago, but it can be toxic. Iodide - it's in iodized salt, so it is clearly less toxic than fluoride.

So I conclude that fluoride is good at preventing tooth decay because it is very toxic, more than other halides present in the body - and the teeth - in larger amounts. Some other halogens are used only as disinfectants.