Families of Element

The lanthanides (or rare-earth elements) and actinides are two families that are related because they both result from electrons being added into an f sub-level. Both families have 14 members, the lanthanides consisting of the elements with atomic numbers 58 through 71, and the actinides including the elements with atomic numbers 90 through 103. However, it is sometimes convenient to consider lanthanum (atomic number 57) as an honorary member of the lanthanide family and to treat actinium (atomic number 89) in a similar manner with respect to the actinides.

The lanthanides are usually found together in the same ores and despite their alternative name of the rare-earth elements, they are not particularly rare. In contrast, only two of the actinides, thorium and uranium, occur in nature, the remainder having been synthesized by nuclear scientists. Members of both families form ions with a charge of +3, although other ions are also formed, particularly by the actinides.