Secondary decay products of thorium include radium and actinium. Four radioactive elements were known in 1900&colon; radium, actinium, thorium, and uranium. The actinide series derives its name from the group 3 element actinium. The actinide series derives its name from the first element in the series, actinium. These decay chains are called the uranium chain, the actinium chain, and the thorium chain. They are commonly lanthanum and actinium, and less often lutetium and lawrencium. Elements 121 and 122 should be similar to actinium and thorium, respectively. The fourth element, either actinium or lawrencium, has only radioactive isotopes. Pa and lighter (i.e. Pa to Pa) is alpha decay, producing isotopes of actinium. Lanthanum and actinium are commonly depicted as the remaining group 3 members. The 4n+3 chain of uranium-235 is commonly called the "actinium series" or "actinium cascade". Due to actinium's intense radioactivity, only a limited number of actinium compounds are known. Ac (actinium and acetyl) and Pr (praseodymium and propyl). The same is true for actinium. Pb is the end of the actinium series from U.