Iodine is very active and kills germs. Xenon is an inert gas used in bright lighthouse lamps. Cesium is the softest metal. It has the largest atoms. Cesium atoms are used in extremely accurate atomic clocks.
Barium absorbs X-rays.
It is used in a liquid swallowed by patients before getting a stomach X-ray. Lanthanum and other lanthanides are used in carbon-arc lamps in motion pictures. Praseodymium and Neodymium are used in welder's goggles. Neodymium is used in astronomical spectrometers. Promethium is used as a power source in spacecraft.