transition metals
Transition metals include the elements that are placed below the periodic table. Those that follow lanthanum La are called lanthanides. They are all shiny, relatively reactive metals. Those that follow Actinium Ac are called actinides. They are all radioactive metals. This means they are unstable. They break down into different, more stable elements. You can read more about radioactive elements in the chapter Nuclear Chemistry . Many of the actinides do not occur in nature but are made in laboratories.