Is Astatine Considered the 18th Rarest Element? No.

We recently received a comment on our blog article, Can You Name All 17 Rare Earth Elements? The reader said: “Astatine 85 is the rarest element. Your list has not mentioned it. So total is 18.”

I’d like to address the reader’s comment.

As mentioned in the article, the 17 Rare Earth Elements (REEs) are as follows in order of Atomic Number:

21 – Scandium (Sc) – does not occur in economic concentrations in the same geological settings
39 – Yttrium (Y)
57 – Lanthanum (La)
58 – Cerium (Ce)
59 – Praseodymium (Pr)
60 – Neodymium (Nd)
61 – Promethium (Pm) – rare and unstable
62 – Samarium (Sm)
63 – Europium (Eu)
64 – Gadolinium (Gd)
65 – Terbium (Tb)
66 – Dysprosium (Dy)
67 – Holmium (Ho)
68 – Erbium (Er)
69 – Thulium (Tm)
70 – Ytterbium (Yb)
71 – Lutetium (Lu)

Astatine (At) may be the rarest naturally occurring element in the Earth’s crust, but it is a member of the halogen family [fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At)] and is presumed to have characteristics similar to other Group 17 elements.

The meaning of the word “rare” in the term “rare earth elements” does not relate to the scarcity of these elements. REE are defined by IUPAC (International Union of Pure and Applied Chemistry) as being the 15 lanthanides (atomic number 57-71) plus Sc (21) and Y (39). These elements are all lumped together as they tend to both occur together and have very similar chemical properties.

I hope this explanation helps to clear the confusion.

Editor’s Note: Here are some additional resources about rare earth elements: