All of these elements are classed in Group 3 because their outer shell holds three electrons. Because they have an odd atomic number, they have few stable isotopes. Scandium, Yttrium, Lutetium, Lanthanum and Actinium can all be found naturally, albeit at very low levels in the Earth's crust, and are collectively called rare-earth metals, along with others in Group 6 and 7. Lawrencium can only be produced in the laboratory and has not been found in nature.