Why do we fluoridate but chlorinate water?

First, I wish to say this question should have been asked in the chemistry forum.

Chlorination:
It is possible to have free un-ionised chlorine, the form of which is used for chlorination.

Fluoridation:
However, fluorine is too reactive to be found in its un-ionised state, hence fluoride. It is impossible to find fluorine at room temperatures. Therefore, it is impossible to fluorinate water. Moreover, it is the dynamics of fluoride ions, rather than fluorine, which mediate in calcium re-crystallization,

The simplest (and oldest) way to chlorinate water is to dissolve gaseous chlorine in it, though various chlorine-containing compounds such as calcium hypochlorite (also known as chloride of lime), sodium hypochlorite, or a chloramine can be used instead.

When you fluoridate water, you don't add gaseous fluorine to it (though if you did, then linguistically speaking, you would be fluorinating it -- however, in practice this is not feasible [see the comments above]); rather, you add a fluorine-containing compound to it.

The oldest method is by adding sodium fluoride (hence the term fluoridation), but today fluorosilicic acid or sodium fluorosilicate are also used.

chlorin-ation
fluorid-ation

simple

http://en.wikipedia.org/wiki/Chlorine

http://en.wikipedia.org/wiki/Fluoride

drop the last "e" - add "ation" - done!