system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If fluorine is such a dangerous substance, why is fluoride, a derivative, not similarly dangerous? How do you safely add fluoride to water, and why is it beneficial for teeth?
gpt
Because compounds and their constituent elements are different.

Hydrogen and Oxygen will, separately, readily combust in a fiery explosion. However, that explosion happens simply because water is way more stable than Hydrogen and Oxygen separately. Once they're bound up in water, they can no longer make that transition from unstable to stable and release a bunch of energy.

Fluorine is dangerous because it is *extremely* reactive. It will react with basically anything. However, in fluoride, the Fluorine has already reacted with something (and then been placed in something that dissolves the compound like water).

To get the slightest bit into the chemistry: Fluorine is extremely electronegative, meaning it is extremely prone to pull an electron off of other chemicals around it. Fluoride is Fluorine after it has already pulled an electron off of something and is much more stable.