Why are lanthanides known as rare earth elements?

Lanthanides are called rare earth elements for historical and practical reasons:

1. Originally thought to be rare: When they were first discovered, these elements were found in very small amounts in certain minerals, so early chemists thought they were rare.
2. Difficult to separate: Lanthanides have very similar chemical properties, which makes them hard to separate from each other. This added to the perception that they were “rare.”
3. Found in specific minerals: Most lanthanides are not found in pure form but are combined in minerals like monazite and bastnäsite, making them less obvious to extract.

Important point: Despite the name, lanthanides are not actually very rare in the Earth’s crust. Some, like cerium, are even more abundant than copper. The “rare” part mainly refers to their historical difficulty of isolation and separation.