Promethium is a synthetic radioactive 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 the only synthetic element in the Lanthanide group as 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 isotope of promethium is ¹⁴⁵Pm, with a half-life of 17.7 years. view more...