Final answer:
Fluorine is the highly reactive, gaseous nonmetal added to toothpastes and water supplies to prevent tooth decay. It forms compounds, like fluoride, that react with tooth enamel to make it more resistant to acids. Fluorine is also used in Teflon coatings and refrigerants.
Step-by-step explanation:
The highly reactive, gaseous nonmetal that is added to some toothpastes and urban water supplies to prevent tooth decay is fluorine (F).
Fluorine compounds, such as fluoride, are added to these products because they have important chemical properties that help protect teeth.
Fluoride ions react with hydroxylapatite in tooth enamel, forming fluorapatite, which is more resistant to acid attacks, preventing dental cavities.
In addition to its anti-decay properties, fluorine is also used in Teflon coatings and as a replacement for chlorine in refrigerants.