Fluorine was a gaseous element, number 9 on the periodic table.
High concentrations of methane, carbon monoxide and fluorine were the typical by-products of Borg assimilation of a civilization on a planet's surface. (Star Trek: First Contact)
In 2154, while trapped aboard the Romulan drone-ship and with their EV suits running out of oxygen, Trip Tucker and Malcolm Reed hastened to find the ship's life support systems. Having already been temporarily abandoned by the Enterprise and having endured a bumpy ride, Reed quipped that with their luck, whatever aliens operate the drone ship probably breathe fluorine. (ENT: "Babel One")
Sensor scans of the Azure Nebula in 2293 determined that it was composed primarily of oxygen and argon with traces of theta-xenon, fluorine and sirillium gas. (VOY: "Flashback")
When the USS Defiant suffered a hull breach after descending into the atmosphere of a class J planet, Lieutenant Commander Dax was nearly overcome after taking several lungfuls of fluorine after the turbolift shaft became flooded with the gas. (DS9: "Starship Down")
When the Delta Flyer got caught in an ion storm and crash-landed under several kilotons of rock on a unnamed Class M planet, the atmosphere surrounding the Flyer was flooded with fluorine gas. (VOY: "Once Upon a Time")