Selenium

Selenium was a non-metallic element found at position 34 in the periodic table. It bore some resemblance to sulfur.

A gas giant investigated by the Enterprise in 2152 had many moons with atmospheres containing selenium isotopes. These isotopes interfered with sensors, hampering the search for Charles Tucker when his shuttlepod was shot down over one of the moons. 

In 2367, Jean-Luc Picard, Wesley Crusher, and Dirgo crash-landed on Lambda Paz, where they encountered an energy sentry guarding a fountain of water. The sentry neutralized threats to the fountain by encasing it in electrically deposited selenium fibers. Dirgo was killed by the sentry in this manner. 

In 2373, Michael Eddington of the Maquis combined large quantities of selenium with rhodium nitrite to produce cobalt diselenide, a poison deadly to the Cardassian nervous system. Eddington released this biogenic weapon on the Cardassian colonies of Veloz Prime and Quatal Prime in the Demilitarized Zone.