Check the listing for details. Neptunium Element Cube. Condition: New. Listed at 53.55 USD. Neptunium Element Cube Made In USA 2x2x2 inches Radioactive symbol. No sample. Neptunium's atomic number is 93 and its symbol is Np. It is is a hard, silvery, ductile, radioactive actinide metal, and can be found in 5 different colors depending on its oxidation state. Neptunium's position in the periodic table just after uranium, named after the planet Uranus, led to it being named after Neptune, the next planet beyond Uranus. Also, it was the first transuranium element, with an atomic mass heavier than uranium, once thought to be the heaviest element. It is normally used to form Plutonium for spacecraft and military applications, but you may find neptunium in your smoke detector. Ionisation-type smoke detectors use tiny amounts of the radioisotope americium-241, but over time this decays into neptunium-237. This display is contained within crystal clear lucite acrylic. Made In USA 2x2x2 inches Radioactive symbol. No sample. Neptunium's atomic number is 93 and its symbol is Np. It is is a hard, silvery, ductile, radioactive actinide metal, and can be found in 5 different colors depending on its oxidation state. Neptunium's position in the periodic table just after uranium, named after the planet Uranus, led to it being named after Neptune, the next planet beyond Uranus. Also, it was the first transuranium element, with an atomic mass heavier than uranium, once thought to be the heaviest element. It is normally uséd to form Plutonium for spacecraft and military applications, but you may find neptunium in your smoke detector. Ionisation-type smoke detectors use tiny amounts of the radioisotope americium-241, but over time this decays into neptunium-237. This display is contained within crystal clear lucite acrylic.eBay integration by