Fluorine is a pale yellow, poisonous gas and one of the chemical elements. It's the most reactive element known to science, meaning it combines with other substances more aggressively and easily than any other element does. Fluorine will react with almost anything it touches, even materials like glass and gold that normally resist chemical reactions.

Scientists identified fluorine in 1886, but working with it proved incredibly dangerous because of its extreme reactivity. For decades, chemists struggled to study it safely. Today, fluorine combines with other elements to create many useful compounds. The fluoride in your toothpaste contains fluorine bonded to other elements; it strengthens tooth enamel and prevents cavities. Teflon, the nonstick coating on frying pans, contains fluorine. So do many refrigerants and some medications.

Pure fluorine remains dangerous and must be handled with extreme care in laboratories. But when chemists bond fluorine atoms to other elements in the right ways, they can harness its reactivity to create materials that resist heat, repel water and oil, and protect our teeth. The key is keeping fluorine locked safely in stable compounds rather than letting it run wild in its pure, gaseous form.