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Chemistry / Re: Why do we add elements to the periodic table that decay instantly?

« on: 21/10/2016 22:54:40 »

Thanks to evan, janus and chiral for all your valuable comments.

The point that "heavy" elements are always getting created in supernovae, is one that I realised with a head-smack after posting.  But I didn't to bother to edit, as it was obviously bound to get picked up.

However, going back to the original question, about whether very transient, swiftly-decaying elements should be added to the Periodic Table.  I would say not, because it would make the list of "elements" too long.

If I might cite a comparable example from Astronomy.  The list of "planets" in the Solar System was eight, until the year 1930.  In that year, a new "planet", Pluto, was discovered.  Right from the time of its discovery, there were doubts whether "Pluto", should properly be added as a planet, as it's very small.  However it got accepted until 2006, when the IAU threw it out. 

This was because a lot of new objects were being discovered, like Eris, which is bigger than Pluto. So if you accept Pluto as a planet, you'd have to accept Eris too. And there were more objects of comparable size.  You could eventually end up with a list of 40 or 50 "planets".  Which is far too long.

So the IAU demoted Pluto to dwarf status.  Now astronomers (well, most of them), only acknowledge eight planets on the list. Pluto and the rest are regarded as interesting, but not genuine.

Couldn't this approach be adopted with the Periodic list of elements?  That's to say,  physicists and chemists, should only recognise 92 genuine elements, the rest being merely interesting?

Because half-life doesn't make a good parameter by which to judge whether something is an element or not.  With Pluto, we didn't have a clear cut definition for what a planet was until the IAU decided on the most recent definition.

We do have a good definition for element.

Besides, using half-life would not be consistent with a cut off at element 92.   Neptunium, Plutonium, and Americium all have Isotopes with half-lives that are longer than the longest lived isotopes of a number of elements with atomic numbers less than 92, the shortest being Americium at 7270 yrs. Compare this to Actinium, Radium, Francium, Radon, astatine and Polonium, The longest lived isotope in this group belongs to Radium at 1600 yrs, and the longest lived isotope of Francium (atomic number 87) is a mere 22 min.
Why include it as a "real" element while rejecting higher numbered elements with more stable isotopes?

222

Chemistry / Re: Why do we add elements to the periodic table that decay instantly?

« on: 20/10/2016 02:40:02 »

Do these new "elements" which last only for seconds or microseconds, exist anywhere else in the Universe, except in laboratories on Earth?

If not, I agree with Luke, they're not real "elements" at all. Just something we've fiddled about with, and cooked up locally.  Surely the fact that they disappear so quickly, rules them out as real elements. How can something that vanishes in a split-second, be regarded as anything more than a terrestrial artefact.

Shouldn't a real element stay around for the duration of the Universe, so that it could always be found in the Universe, without relying on a terrestrial source?

Many "Natural" elements do not have infinite lifetimes.  Carbon 14 is constantly forming being formed in our atmosphere and decaying, Uranium is also a "naturally" occurring element which decays, and in doing so creates other short-lived elements. I don't think it makes much sense to say that naturally occurring Radium is not a "real element" just because its most stable isotope has a half-life of just 1600 years.   
As far as these other element existing elsewhere in the universe, If we can make them here on the Earth, then surely they are formed in the supernovae that forged all the other elements including the Uranium we still find in our own crust.   
That's one reason for trying to make these elements on Earth; to learn the rules that determine just what is possible in terms of element formation and to verify whether or not our understanding of what these rules are stand up to experiment.