Neon
Neon is an element which was gaseous under standard conditions. It could be used in neon signs.

In 2267, the  crash landed on Taurus II. Doctor Leonard McCoy considered the planet's atmosphere breathable, which was an oxygen-nitrogen mixture, with acceptable quantities of neon, argon, and krypton. ()

Later that year, the  was forced to land on a Gamma Canaris N. The crew found the atmosphere to also be an oxygen-nitrogen compound very similar to that of Earth, again containing trace amounts of neon, argon, and krypton. ()

In 2365, the atmosphere of Theta VIII contained 14% of liquid neon. ()

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Elements