What is the rarest element on Earth?

jbean

Astatine. Less than 75 mg in earth's crust Feb 11 04, 9:19 PM

sequoianoir

Apparently although there are 20 known isotopes, none of them are stable and the longest lived, astatine-210, only has a half-life of 8.3 hours. It "appears" from the minor branching in the decay of uranium, thorium and actinium, and disappears just as quickly it seems. It is also soluble which may contribute to this. Astatine is a halogen and possibly accumulates in the thyroid gland like iodine. Feb 13 04, 9:01 AM

ephtan

Francium. Only 30 grams of it exist in the earth's crust at any one time, in Uranium deposits. http://www.madsci.org/posts/archives/nov99/942610560.Ch.r.html [Link added -- Zb] Jul 06 08, 6:54 AM

sequoianoir

Astatine with just 25g according to the Guinness Book of Records as answered to this other FT question! http://en.wikipedia.org/wiki/Astatine Also see Question #92497. http://www.funtrivia.com/askft/Question92497.html Jul 06 08, 6:14 PM

karozans

Well I am no chemical engineer but Astatine and Francium is probably correct if you are assuming that the element actually exist naturally on earth. For instance Technetium (43) and Promethium (61) have atomic numbers that are next to elements that are quite abundant like Molybdenum (42) and Neodymium (60). So in a sense one could argue that these elements should exist naturally on the earth however they do not. These two elements 43 and 61 can only be synthetically prepared just like elements larger in atomic number than Uranium (92). So I guess technically Astatine and Francium is the rarest natural element on the earth that exists. Technetium and Promethium should exist but they don't, so in a sense that would make them rarer. Astatine and Francium have been produced synthetically but it is hard to do because of the short half-life. So only a about 10,000 atoms can be created at a time. Technetium and Promethium can be created in abundance though. Apr 22 09, 5:37 AM

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