Fluorine
Fluorine is 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. ()

In 2154, while trapped aboard the Romulan drone ship and with their EV suits running out of oxygen,  and Malcolm Reed hastened to find the ship's life support systems. Having already been temporarily abandoned by the  and having endured a bumpy ride, Reed quipped that with their luck, whatever aliens operated the drone ship probably breathed fluorine. ()

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. ()

When the  suffered a hull breach after descending into the atmosphere of a Class J gas giant in 2372, Lieutenant Commander Dax was nearly overcome after taking several lungfuls of fluorine after the turbolift shaft became flooded with the gas. () 

In an alternate timeline where the Borg assimilated Earth after stopping Zefram Cochrane's warp flight, the planet had an atmosphere of methane, carbon monoxide, and fluorine in 2373. ()

In 2375, when the  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. ()

See also 
 Hexafluorine
 Hydrogen fluoride
 Trifluorinate compound

External link 
 

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Elements