An elemental question

Is there an easily-explained reason why there is apparently an upper limit on naturally occuring elements? Or is there not really one and the reason elements higher on the periodic table than plutonium are not found naturally entirely a function of decay?

The stability of elements, and the energy budget required for their formation places inescapable limits on the proportions of each element in the universe.

That proportion means that the elements beyond 92 are extremely rare, and for the most part extremely ephemeral.

There is some created, in some supernova somewhere, and a few minutes later, it no longer exists.

Tris

Basically their half-lives are much shorter than the age of the earth so any of those elements that may have been present have long since decayed.

Plutonium is barely available naturally by the way. Some is found in uranium ore because the uranium, being radioactive, makes some plutonium but by and large it is pretty rare naturally and needs to be manufactured.

Essentially, what Tris said. Any quantity of transuranic elements that may have existed “at the formation of the Earth” (ignoring the fact that that was a process measured on a geologic time scale) will have broken down. In fact, there are four “actinide breakdown series,” three of which alpha and beta decay convert Uranium 238 and 235 and Thorium to three isotopes of lead, and with the longevity of those three nuclides, quantities of each continue to be available, as well as at least traces of the intermediate breakdown products. But the fourth series, starting with Neptunium 237, has completely gone through its breakdown process and the end product, Bismuth, is all that remains.

Besides the trace quantities of Neptunium and Plutonium 239 in uranium ores, there is some indirect evidence for Californium 257, with a 50-odd-day half-life, being present in supernovae and their immediate aftermath.

It might also be noted that two elements fairly low on the periodic table: Promethium and Technetium, have no stable or metastable isotopes, and hence do not exist in nature. And of the elements between Bismuth and Thorium, one, IIRC Francium, has only isotopes of such a short half-life that the total naturally occurring quantity of it on Earth is estimated to be measured in a few grams.

I typed out a long quote and lost it, and I am not retyping it.

But the gist was that the nucleus is held together by the strong force, a force that works only at ridiculously tiny distances. Larger nuclei cannot be held together as well as smaller ones, and the breakdown point appears to come at around 100 protons. This is the cause of the short half-lives people are talking about.

The fact that the force tending to break up the nucleus (repulsion between like charges) obviously grows stronger as the number of protons increases. That’s why stable and metastable isotopes have a higher neutron/proton ratio as you move up the list (neutrons contribute to the strong force, but not to the repulsion).

It’s not just the number of protons; [thread=332248]here[/thread]'s an old thread in which is discussed this exact issue, but in essence it has to do with the geometry of the nucleus and the ability of nuclear particles (protons and neutrons) to interact via the residual strong force. If that sounds a bit vague, well yeah, it is. The answer to the OP’s question is no, in fact there is no easily-explainable reason why there is an apparent upper limit. You’d have to study quantum chromodynamics to gain an understanding of what holds nuclei together to begin with, and once you did so you’d realize that physicists, in fact, don’t have a, er, strong grasp on why nuclei break down at larger sizes or why there seems to be a virtual dead end at an atomic number of three figures. It’s all just part of the general weirdness of this bizarrre universe in which we live.

Stranger

All true, but there are some theories that there may be some additional islands of stability further up - beyond where we’ve been able to go with available atom smashers. Anyone know what new particle accelerators might be coming online in the next few years?

“Islands of stability” refer to elements that might have incredibly long half-lives - up to a few seconds, conceivably.

So they’re not really relevant in this discussion, as the term “stable” is relative and they’re not stable enough to be useful for anything.