Plutonium was the second transuranic elem ent of the actinide series to be discovered. By far of greatest importance is the isotope 239Pu, which has a half-life of more than 20,000 years.
Americium is a synthetic radioactive chemical element with the symbol Am and atomic number 95. It is a transuranic member of the actinide series, in the periodic table located under the lanthanide element europium, and thus by analogy was named after the Americas.