Period 6 element period 6 other metals are incredibly toxic, such as thallium. Period 6 contains the last stable element, lead. All subsequent elements in the periodic table are radioactive. After bismuth, which has a half-life or more than 10 years, polonium, astatine, and radon are some of the shortest-lived and rarest elements known; less than a gram of astatine is estimated to exist on earth at any given time. Caesium or cesium is the chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of 28 °C (82 °F), which makes
Heavy metals with the term "heavy element". For example, in discussing the history of nuclear chemistry, Magee notes that the actinides were once thought to represent a new heavy element transition group whereas Seaborg and co-workers "favoured ... a heavy metal rare-earth like series ...". In astronomy, however, a heavy element is any element heavier than hydrogen and helium. In 2002, Scottish toxicologist John Duffus reviewed the definitions used over the previous 60 years and concluded they were so diverse as to effectively render the term meaningless. Along with this finding, the heavy metal status of some metals is occasionally challenged on