Tuesday, 16 October 2012

How many naturally-occurring elements are there?

The editor of Nature Chemistry, @stuartcantrill and I follow each other on Twitter.  If memory serves, that followed from a quick exchange about the correct value of the number of naturally-occuring isotopes between us.  The subject has come up again, with Stuart (correctly) correcting the value given in a blog post he tweeted about.

The question is not at all easy to answer.  It depends what you mean by naturally-occurring.  I think the common meaning is "can be dug up from the ground but didn't come from man-made sources such as weapons fallout."  So, how many is that?  Well, definitely any element with a stable isotope can be included there.  How many elements have stable - i.e. non-radioactive - isotopes?  If you look at a table of isotopes, such as this one you can look for the squares shaded in black, which is the usual notation for a stable isotope.  That gives us everything from Hydrogen (element 1) up to Bismuth (element 83) with gaps for Technitium (element 43) and Promethium (element 61).  The complicated thing is that really, everything heavier than around Nickel or Iron (around element 26) is know to be theoretically unstable, and in fact the decay of the only "stable" Bismuth isotope has been observed.

Still, it turns out that all the isotopes listed as "stable" in the chart can be dug out of the ground, and trace amounts of Technetium and Promethium are found in nature, because Uranium can be dug out of the ground, and though all isotopes of it are radioactive, they sometimes decay by spontaneously fissioning into lighter isotopes, which include Technetium and Promethium.

It is for similar reasons that some other radioactive-only isotopes can be found on the Earth, and this includes isotopes of all the elements heavier than Bismuth but lighter than Uranium (things like Polonium and Radon).  It seems that a bit of Neptunium and Plutonium (elements 93 and 94) can be found in Uranium ores, too.  According to this page found by Stuart, the number may be even higher, though I'd like to see the references to the papers where the things heavier than Plutonium were observed in non-man-made matter.

In some ways, though, the question is a bit academic.  If we extend our remit to the stars, where all elements heavier than lithium are made, then surely even heavier elements than those that can be dug out of the ground are found - at least fleetingly during novae and supernovae.  Stars are part of nature, after all.  What is clear is that we know that at least 118 elements exist in isotopes for long enough to say that they exist.  We also know, or suspect, that there will be an upper limit to this number, but exactly where that is, we're still trying to work out.