Chemical Elements

A question about the periodic table and the prevelance of “Natural” chemical elements.

Is there a reason there are only 92? Is there some physical law or scientific rationale which determines an element shall have no more than 92 protons, or whatever - or is it just a question of us not having discovered a stable, naturally occuring #93

The second, mostly, although of course there’s underlying physical laws as to “why”. After all, note that for some natural elements there’s isotopes which are stable and others which aren’t.

There are predictions about the lifespan of some high-number element’s isotopes which, if they turn out to be confirmed, would confirm much of what we know about nuclear physics and the mechanisms of disintegration; if the predictions turn out to not work, the theories will need to be tweaked.

In theory, there could also be extreme locations (brown dwarves, say) where some of those elements which don’t exist naturally on Earth do exist. Those isotopes that have short lifespans on Earth would still have short lifespans there, though. Remember that a star is basically a huge fusion reactor, in one dense enough you could get heavier elements than we get in the Sun.

There are 94 that occur naturally on Earth. Only 80 have stable isotopes.

The number itself is the number of Protons in the nucleus, so that number 93 already exists and just is not stable.

The Science community is working on verification that element 122 occurs naturally in Thorium deposits.

Consider a graph showing the number of protons on the horizontal axis and the number of neutrons on the vertical.

IIRC there are theoretical reasons to expect the most stable atoms to lie close to a line that is not quite at the 45° angle, such that there are generally 0 or 1 or 2 more neutrons than protons, and to expect a long region from the origin outward where things are stable, and then the stability starts wavering up and down, so that there ought to be a couple of islands of stability further out this line with no stable elements bridging the islands to the main line.

And in fact there are no stable atoms bigger than Helium that have more protons than neutrons.

Also, there are special reasons having to do with whether the number of protons is even, and whether the number of neutrons is even. Almost 2/3 of all kinds of atoms (all isotopes) have even numbers of both protons and neutrons. There are no stable isotopes heavier than Nitrogen that has equal odd numbers of both. For most fission fuels, the number of protons is even and the number of neutrons is odd.

This is excellent, entertaining, accessible, and short!

http://www.phys.psu.edu/~strikman/49705/lecture17.pdf

There’s a very delicate system of checks and balances in place in an atomic nucleus that determines its stability – basically, a nucleus is stable only if it can’t reach a configuration of lower energy via some form of radioactive decay. Simplified, the main opposing forces in the nucleus are repulsion via the coulomb (electrostatic) force between the protons (like charges repel), and attraction via the strong nuclear force that acts between the protons, the neutrons, and each other (that’s basically why a nucleus always needs more neutrons to be stable – they’re sorta used as glue).

So, the more protons you got, the more neutrons you need, seems easy enough. But at some point, you run into trouble; omitting some details, it’s sometimes convenient for a neutron to turn into a proton via beta (-) decay, i.e. the emission of an electron and an electron-antineutrino. This happens, basically, when there are far more neutrons than protons (essentially, the neutron can occupy a lower energy level if it turns into a proton, lowering the total energy, so it just goes ahead and does that), so, at some point, you can’t add any more neutrons to keep the nucleus glued together, since those’d just turn into protons.

Thus, we get an effective maximum number of protons we can have in a stable nucleus – somewhere around 83, which means bismuth (though bismuth actually does decay, but it takes a heck of a long time to do so, so we can consider it stable for all practical purposes).

Orders of magnitude less than 1% of the Earth, by mass, is in atoms with an even number of neutrons. Even in the thin crust, where we are and where they are favored, they are still only 1%.

This has nothing to do with the OP’s reasonable hunger for a reason why atoms should get no bigger than they do; it just seemed nifty. Sorry.

Could you please clarify that a bit? Hydrogen has only 1 proton, an odd number, and it’s the most abundant element in the universe.

*** Ponder

For the unstable ones, it just comes down to lifespans, and what sorts of processes create them. For instance, plutonium (in its stablest isotopes) has a much shorter lifespan that uranium (in its stablest isotopes), so there’s still some uranium left over from the time of the beginning of the Solar System, but any plutonium that was formed at that time is long since gone. There’s still traces of plutonium, but from processes which created it much more recently.

>Could you please clarify that a bit? Hydrogen has only 1 proton, an odd number, and it’s the most abundant element in the universe.

I said almost 2/3 of all KINDS OF atoms, not of all atoms. A hydrogen having one proton and zero neutrons is only a single kind, though perhaps 90% of all atoms in the universe are of this kind.

I was going to mention this. The work was origionally published from a group in Isreal. From what I understand though, it is still a freak incidence of fusion happening naturally and the lifetime of the element is still fleating.

Here is the wiki. Sounds like the research is a bit sketchy for the moment. If they turned it down without peer review the research is probably flawed fundametaly.

Minor correction to Jim’s (What Exiut) correction of the OP.

The number 92 is correct. But it’s not elements with atomic numbers 1-92.

Trace amounts of francium (#87 IIRC), neptunium (93), and plutonium (94) do exit naturally, as noted in passing by Chronos.

But the same is not true for promethium (43) and technetium (61). So the 92 are ##1-42, 44-60, 62-94.

As long as we’re limiting our discussion to the Earth and Solar System.


And even then, Pm does exist naturally–but just an atom at a time for an hour or two, perhaps–in Uranium ores:

http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B758S-48M3FD8-FX&_user=789896&_coverDate=05%2F31%2F1968&_alid=794985242&_rdoc=2&_fmt=high&_orig=search&_cdi=12934&_sort=d&_docanchor=&view=c&_ct=2&_acct=C000043360&_version=1&_urlVersion=0&_userid=789896&md5=e22538a782b22949fe4498ee4bbac456

doi:10.1016/0022-1902(68)80427-0

Nope. Natural nucleosynthesis due to stellar fusion produces elements up to [sup]65[/sup]Ni; heavier than that and it has to be produced by one of the processes of supernova nucleosynthesis (in r-, p-, and rp-process). In brown dwarfs, fusion occurs only marginally and intermittently if at all; such bodies produce most of their energy by gravitational collapse and internal radiodecay.

Stranger

Is there a graph like this on the net somewhere that you know of?

>Is there a graph like this on the net somewhere…

Pages 18, 19 and 20 of the ref I gave above and give again below are just such graphs:

http://www.phys.psu.edu/~strikman/49705/lecture17.pdf

They put protons on the vertical axis whereas I put it on the horizontal, but this difference is superficial.

The excellent graph at:


is another such graph.

Generally, what you’re looking for is a map of nuclides; here is a somewhat humongous example (around 4MB). Number of protons is on the vertical, neutrons on the horizontal axis, so all nuclides in one row are isotopes (equal proton number) of one element, all in one column are isotones (equal neutron number), and all along the upper-left to lower-right diagonal are isobars (equal mass number).

It’s especially useful for tracking decay chains, since it notes which kind of decay any given nuclide undergoes (in this case, yellow for alpha, blue for beta+ and pink for beta-; gamma decay doesn’t result in nuclear transmutation, being just the emission of electromagnetic radiation).
To find out what the end product of any given decay is, all you have to do is count:

For alpha decay, which is the emission of a [sup]4[/sup][sub]2[/sub]He - nucleus, the decay is:
[sup]A[/sup][sub]Z[/sub]N —> [sup]A-4[/sup][sub]Z-2[/sub]N* + [sup]4[/sup][sub]2[/sub]He
Meaning that to find the daughter nucleus of an alpha-decaying nuclide on the map, you go down two rows, and left two columns, corresponding to ‘losing’ two protons and two neutrons.

For beta+ decay, emission of a positron (and an electron-neutrino, but that doesn’t change anything), the decay is:
[sup]A[/sup][sub]Z[/sub]N —> [sup]A[/sup][sub]Z-1[/sub]N* + e[sup]+[/sup] ( + v[sub]e[/sub])
So, on the nuclide map, you go down one row, and right one column, corresponding to one proton being transformed into a neutron.

For beta- decay, emission of an electron (and an electron-antineutrino), the decay is:
[sup]A[/sup][sub]Z[/sub]N —> [sup]A[/sup][sub]Z+1[/sub]N* + e[sup]-[/sup] ( + !v[sub]e[/sub])
So, on the nuclide map, you go up one row, and left one column, corresponding to one neutron being transformed into a proton.