The lanthanides (lanthanoids) have atomic numbers from 57 through to 71. The geochemical behaviors of lanthanides are a regular function of ionic radius and atomic number. In the lanthanide contraction, the ionic radii of the lanthanides decreases through the series. This relationship between geochemical activity and ionic radius results in variations in the abundances of lanthanides that can be used to trace natural materials through physical and chemical processes. The lanthanides are divided into light- and heavy-lanthanide enriched minerals because of the lanthanide contraction. The light lanthanides are more abundant in the Earth's crust, whereas the heavy lanthanides are more abundant in the Earth's mantle. As a result, large rich orebodies are much more likely enriched in the light than the heavy lanthanides. (The lanthanides obey the Oddo-Harkins rule, according to which odd-numbered elements are less abundant than their even-numbered neighbors.) Also useful to geochemists, 147Sm and 176Lu are radioactive isotopes with long half-lives that can be employed in radiometric dating. The lanthanides, except for Ce and Eu, occur as trivalent cations in nature. The lanthanides are often referred to in geochemistry by the trivial name, "rare earths". REE is sometimes used to describe all the lanthanides except Pm, together with Sc and Y. The term "rare earths" came into use because the lanthanides were isolated from uncommon oxide-type minerals. The term "earths" is an obsolete term that described the water-insoluble strongly basic oxides of electropositive metals that late 18th century technology was incapable of smelting to metal. Although these elements are found only in trace amounts compared to the common rock forming minerals, they are not rare: Nd is more abundant than gold, and even the least common naturally-occurring lanthanide (Tm) is more abundant than iodine. Here's a mnemonic for the lanthanide series: Ladies Can't Put Nickels Properly into Slot-machines. Every Girl Tries Daily, However, Every Time You Look. | 
