Promethium is a chemical element that has the symbol Pm on the periodic table. It has the atomic number 61 which means it has 61 protons in its atom. It is a Lanthanide. All its isotopes are radioactive. Together with Technetium, these are the only two radioactive elements below lead. Most Promethium is artificially produced. The longest-living promethium is Pm, with a half-life of 17.7 years. 