Is there some kind of law that prohibits atoms from becoming any bigger than what’s found along the bottom of the Periodic Table? Or is it more of a lack of sufficient energy that causes an atom to break down to something smaller and more stable?
There are current theories about islands of stability beyond the current elements in the periodic table, but thus far, AFAIK, all the artificially-synthesized heavier elements disintegrate within minute fractions of a second.
It’s not that there’s an upper limit on the size of an atom, just that larger atoms tend to have shorter lifespans before they decay. One can, in principle, make atoms heavier than the ones in your periodic table, but they’re likely to decay in a matter of microseconds. Theoretically, there should be some heavy atoms which lie in so-called “islands of stability”, which would last much longer than other atoms of similar mass, but even there, you’d probably only have lifetimes on the order of a few seconds.
Also, I may be oversimplifying here (or talking out of my ass,) but atomic number is determined by number of protons in the nucleus, and protons all have a positive electrical charge. The more protons you cram into such a small space, (as it were,) the greater the instability of each of them wanting to get the hell away from all those other positive charges.
IIRC the stuff about the islands of stability might have to do with the behaviour of the weak nuclear force, which binds nuclei together against the electromagnetic force.
Isnt there a predicted “island of stability” around the 115 to 120 mark?
According to what I’ve read, nuclei are held together by residual charges from the strong force, which holds quarks together to form protons and neutrons. The weak force is something else.
While I’m not an atomic physicist, I think this is very close to being an accurate summary of the situation. With the exception of (normal, light) hydrogen (AKA protium), every nuclide (“isotope”) has a nucleus composed of one or more protons and one or more neutrons. The neutrons serve as a “buffer” to stabilize the nucleus against the proton-proton electromagnetic repulsion. The ratio of neutrons to protons in a stable or metastable nucleus varies from 0.5 in Helium-3 (“tralphium”) to about 1.6 for the stuff at the top of the periodic table.
Apparently, the maximum number of protons that can cohabit in a nucleus without undergoing radioactive decay in any measurable amount is 83, i.e., Bismuth, which has a stable isotope with 83 protons and 126 neutrons. However, metastable nuclides with half-lives a reasonable percentage of geological time exist at 90 and 92 (thorium and uranium; IIRC Np-237’s half-life is also measured in millions of years, though not long enough for any measurable amount to remain.)
Above an atomic weight in the high 250s, there is a strong tendency to undergo spontaneous fission in a microsecond-range time frame. Below that, spontaneous fission may occur but the more common breakdown methods are the alpha and beta decay familiar to anyone interested in radioactive decay.
There are reasons founded in physical structure (e.g., the “droplet” model) why these issues should be so, but I am not clear enough on them to essay trying to discuss them.
Um…sort of. The internuclear force or residual strong force is a result of exchanges between nuclear particles. Incidently, the forces are similar regardless of whether the particles are charged (protons) or uncharged (neutrons). The strong interaction is a result of exchanges of quarks between nuclear particles that are controlled by gluons in order to maintain color charge (not related to electrical charge). A simple conceptual model of decay in an atomic nucleus envisions a lump of nuclear particles which stick together via these interactions; once the nucleus gets too large or is of a configuration in which the particles can’t form a shape that lets the particles “touch” each other, there is a certain probability that one or more of the particles will break loose (alpha decay), or will cause a proton to convert into a neutron releasing an anti-neutrino and electron, or convert a neutron into a proton releasing a neutrino and a positron (beta minus and beta plus decay, unless I screwed up and got it backwards.) The worse the geometry, the greater the chance of decay. This model isn’t literal–the particles all exist as probability waveforms, not little bits of stuff–but it gives you the general idea of why large atoms, or those with the wrong number of neutrons, decay. To gain a true understanding of it, or rather, an understanding of the most accepted model we have to date, you’d have to spend some number of years studying quantum chromodynamics, only to learn that we still don’t fundamentally understand what is going on with quarks and gluons, although we can talk about the effects with lots of intimidating math.
The weak force is what causes beta decay; it affects neutrinos and, according to electroweak theory, is a different manifestation of the electromagnetic force over extremely small distances, though I don’t really have a good understanding of how that works.
There may not be a maximum size for a nucleus, but there certainly seems to be a physical restriction as far as getting past unstable configurations. It’s kind of like building a geodesic dome; the larger you build it, the more stable a completed dome is, but at some point in scaling the intermediate structure becomes so unstable that it’s impossible to make it any larger (without providing additional support). There is the suggestion that well past the current transuranic elements and past the proposed islands of stability may lie some genuinely stable elements with massive atomic weights…which begs the question as to where these things are and how is it we’ve never found one. More than likely the claims are either nonsense or are predicated on truely enormous energies and pressures which aren’t found anywhere in the Universe when/where normal baryonic matter can exist.
Stranger
Thanks for the replies everyone. I won’t pretend to completely understand it, but I can at least narrow down my reading list to gain a deeper understanding.
Also, the nuclear force which holds protons together works only over very small distances. The more protons, the bigger the nucleus, and the greater the distance.
IANANP; I just read this in Bill Bryson’s "A Brief History of Everything"
Sort of. The strong force, which holds quarks together into protons and neutrons, actually gets stronger with distance. If you tried to separate two bound quarks, the energy required would grow with distance–it is the opposite of gravity. Separate two bound quarks far enough and the energy is so great that two quarks will appear out of the quantum foam to form meson pairs with the separated quarks.
What you are talking about, the residual strong force that holds the nucleus together despite electromagnetic repulsion, indeed does work only over very small distances within the nucleus. Since nucleons (protons and neutrons) have no color charge, the full effects of the strong force (exchange of gluons) do not apply. The distinction between the strong force and residual strong force is particularly important.
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I believe there is an “Island of Stability” that’s been predicted to be in the vicinity of 115 to 120.