Rare gases refers to the elements helium, neon, argon, krypton, and xenon. They are rare in that they are gases of very low density (“rarified”) at ordinary temperatures and are found only scattered in minute quantities in the atmosphere and in some substances. In addition, rare gases have zero valence and normally will not combine with other elements to make compounds.

Rare Earth elements are elements numbered 58 through 71 in the periodic table plus yttrium (Y, element 39) and thorium (Th, element 90). They are called “rare Earths” because they are difficult to extract from monazite ore, where they occur. The term has nothing to do with scarcity or rarity in nature.