Final answer:
Rare earth elements refer to 17 elements, including the 15 lanthanides plus scandium and yttrium, which are known for their similar chemical properties and historical difficulty in economical extraction and separation.
Step-by-step explanation:
The rare earth elements are a collection of 17 elements, including the lanthanides, scandium, and yttrium, that often occur together and have similar chemical properties, which makes their separation challenging. These elements are set off from the main body of the periodic table and are sometimes referred to by the name of one of its members. The lanthanides refer to the elements with atomic numbers 57 through 71, which are found in the sixth period of the periodic table and are part of the 'f-block' of elements. Scandium and yttrium, while not part of the lanthanide series, are also considered rare earth elements due to their similar properties and are commonly found with the lanthanides in various mineral ores.
The term 'rare' is historical, as these elements were once hard to extract economically and it was difficult to obtain a pure sample. However, they are not necessarily rare in the earth's crust. Modern separation techniques, such as ion exchange resins, have made it more economical to separate these elements. The rare earth elements are significant in many modern technologies and are used in the manufacture of a wide range of devices, including electronics and renewable energy systems.