The halogen of the week: Fluorine. Symbol is F. Atomic number is 9. It is the element for which the largest number of atoms combine with atoms of other elements..."univalent"....it readily forms compounds with most other elements...It is the most chemically reactive and electronegative of all the elements. 

In pure form it is a poisonous pale yellow/brown gas. Like the other halogens , contact with skin gives you severe chemical burns. It is a powerful oxidizing agent and reacts EXPLOSIVELY with hydrogen. It is too reactive to be found in elemental form.

Fluorides are compounds that combine fluorine with some positively charged counterpart.

--atomic fluorine is used in semi-conductor manufacturing

--sodium fluoride compound and others are used in toothpaste and water fluoridation,  more        about that controversy later....

--SSRI anti-depressants are fluorinated molecules! And because our bodies have difficulties metabolizing fluorinated molecules, these are the major cause of fluorinated organics found in sewage and waste water!

--Fluorochlorohydrocarbons are used in air conditioning and refrigeration. CFC's, or chlorofluorocarbons, are banned due to ozone destruction, but it is the chlorine and bromine radicals which are the guilty parties, not fluorine, thus hydrofluorocarbons are not banned by the EPA. They have a relatively small greenhouse effect compared to CO2 and methane.