Among nonmetal elements, flourine is the most reactive. Element reactivity is determined by reading the periodic table of elements; for nonmetals, reactivity increases as you go up a group and move from left to right across a period.
Since flourine cannot be separated from any of the compounds it belongs to by using a chemical, it is very hard to isolate. Flourine compounds have been in use since 1670, when it was used to etch glass. George Gore was the first to produce flourine as a separate element through an electrolytic experiment in 1869.
Some cities add flourine to the water supply to help prevent tooth decay. It is also a common component in toothpaste.