Named for its Norse mythological counterpart and offering potential as a nuclear fuel, Thorium is periodic element number 90.
Discovered in 1828 by Reverend and Esmark, it has a silvery metallic appearance and will tarnish to black upon exposure to air. Each of its seven natural isotopes are weakly radioactive, which led to ceasing of its use in gas mantles and alloys. Thorium is commonly used in the manufacture of high-end optical systems and scientific instruments, and may come to replace uranium in powering reactors. However as of 2016, only several Thorium reactors exist so we have quite a way to go just yet.