So is there a form of cobalt with 27 protons and 27 neutrons? If so, is it stable?
It’s fairly easy to create neutron-rich isotopes in nuclear reactors that undergo beta decay by emitting electrons. But how do they create neutron-poor/ proton-rich isotopes that either emit positrons or capture electrons?
Here’s cobalt-54 (27p+27n) on the table of nuclides. Its half life is about two-tenths of a second.
While neutron capture is at the top of the list of ways to make a new isotope, there is the question of why the neutrons are there in the first place to be captured.
Most of the long-lived isotopes you find now were around when the Earth was first forming from the cloud of matter that resulted from the supernova death of a second generation star. That star’s nuclear furnace had free neutrons running around all over the place generating all manner of isotopes.
On Earth today, you can get free neutrons from:
- Other decaying isotopes. (Nuclear reactors result in a lot of this, but it happens in nature as well.)
- Accelerator systems. (If you smash protons into stuff, you can knock out free neutrons.)
- Cosmic rays. (High energy cosmic ray muons knock neutrons out of nuclei in the atmosphere. These neutrons get captured by N-14 which then loses a proton to make C-14. This C-14 is what gets used in carbon dating and is also useful in medical radiotagging.)
Neutrons aside, pretty much anything that you hit a nucleus with can, in principle, change it to make something radioactive. Protons, alpha particles, neutrinos, pions… you name it – if you can induce a reaction in the nucleus, you can make a new isotope. (To be sure, some things induce changes more easily than others, and the energy of the bombarding particle is very important, too. Fast neutrons don’t do as well as slow ones, for example.)
There is also the Wigner Effect. This is the result of radiation impacts displacing atoms within the materials crystal structure. This can cause significant levels of energy to be stored in the material without changing the isotopic composition.
This was a problem with early graphite moderated reactors - if the graphite was not heated up to re-anneal the graphite and release the stored energy, a catastrophic wigner energy release could occur and set it on fire. Of course, the annealing process was risky as well, raising the reactor heat and risking fire.
Si
Interesting. In high school, I got the impression that the “normal” state of an element was an equal number of protons and neutrons with electrons scattered througout the various shells to balance the charge. According to this thread, isotopes are the most common form of some elements and the equal-number protons and neutron form is unstable while some isotopes are stable.
Is this common amongst elements?
“Isotope” refers to all forms an element, from most common to least. Because of the association some people have with the term and radioactivity or “abnormalness”, the term “nuclide” is preferred by some.
At the low end of the periodic table, starting with Helium, having an equal number of protons and neutrons is most common. But as one goes up, the proportion of neutrons needs to go up so that the strong force holding the nucleus together can balance out the electromagnetic force trying to pull it apart.
See the chart of the nuclides linked to above by Omphaloskeptic.
The really interesting thing about stability is how having an even number of protons and/or neutrons affects things.
What ftg said.
“Isotope” does not mean “equal number of protons and neutrons.” An isotope simply refers to the different forms of an element (which by definition must have the same number of protons, or the element would change), but with different numbers of neutrons. The differing number of neutrons changes the atomic weight of the different isotopes.
The word “nuclide” is used as a synonym for “isotope.”
By the time you get to the very heavy elements, it takes more and more neutrons to hold an atom together. For example, the most common isotope of lead is Pb-208, which has 82 protons, and 126 neutrons.
The isotope of lead with the least number of neutrons ever detected is Pb-178 (82 protons and 96 neutrons). Its half-life is just 0.23 milliseconds. Pb-164 would have 82 protons and 82 neutrons, but it has never been observed or created in the lab. If it were ever created, it’s half-life would orders of magnitude shorter than that of Pb-178. It could be in the nanosecond range, picosecond range, or even shorter.
I guess this is the key fact underpinning my ignorance. I did not know that neutrons help hold the nucleus together.
Or, more likely, it’s not a bound state at all.
Hypno-Toad: Definitely play around with a table of nuclides. Here’s the one I like: Table of nuclides. Just click anywhere on the chart to sort of zoom in. The number of protons increases as you move up the chart, and the number of neutrons increases as you move to the right. Clicking on any nuclide tells you loads about it.
Indeed. Protons attract each other a little bit due to a sort of quark-based “covalent” bond (the strong nuclear force), but they much more strongly repel each other due to their positive electric charge. Neutrons, on the other hand, provide the same attractiveness without any repulsiveness from electric charge. Stuffing in neutrons both spaces the protons apart a bit and adds glue through the strong nuclear force.
I was looking at that chart earlier and man, it makes the period table look like book on ABCs.
Pasta, so does that mean that it’s the protons’ repulsion that causes nuclear instability in the first place?
So do different nuclides of the same element have different chemical properties, or is that all in the electron bonding? And what might happen if a radioactive element in a chemical compound should decay? Does it suddenly react differently with the other elements with which it is bonded?
The chemical properties of isotopes are very, very similar, but they are detectable, especially with lighter elements. The most notable example is deuterium, which behaves quite a bit differently than hydrogen. When a radioactive atom in a molecule decays (alpha decay), the molecule will probably fall apart, because the number of electrons in the atom will immediately change.
For fixed total nucleons (p+n): On the low-neutron side of “stable”, it’s the proton repulsiveness that causes instability. On the high-neutron side of stable, it’s the neutron’s instability that cause the instability. The neutron is itself an unstable particle. Within a nucleus, though, it gains stability by being in a bound state (i.e., lower energy state). But if you keep stuffing in neutrons, eventually you stop overcoming the underlying instability of the neutrons, and you get beta decay.
For the most part, changing the number of neutrons does not effect the chemical properties (in any significant way). The exception is hydrogen (and to a less biologically important degree, helium). This is just because hydrogen is so light to begin with. Hydrogen-2 weighs twice as much as hydrogen-1, so higher-order contributions to bond strenghts are changed enough to be detectable. You can throw critical biological processes out of whack, both from the slight change in bond strengths and from the extra mass that slows down cellular transport processes, by replacing enough [sup]1[/sup]H with [sup]2[/sup]H.
Your intuition is correct about decay within a compound. Except for gamma-ray emission, a radioactive decay would result in a different element, and the compound would be no more.
For some values of “quite a bit”. It’s detectable, certainly, but most of the differences are down in the tenth of a percent range.
Yeah, adding too many neutrons doesn’t help. And it doesn’t take that many. With Hydrogen, adding just two (Tritium aka Hydrogen-3) gets to an unstable isotope. With Helium, ordinary Helium-4 is already at a maximum*. For both, adding more neutrons doesn’t give you an isotope long-lived enough to be anything but of theoretical interest.
*But adding 2 or 4 is better than adding 1 or 3. Even is good in nuclei.
Well, it won’t support life, so that’s “quite a bit” in my book.
So what happens to a neutron not in a nucleus? Does it break down into quarks?
Free Neutrons have a half-life of around 15 minutes, decaying into a Proton, Electron and an anti-Neutrino:http://van.physics.uiuc.edu/qa/listing.php?id=1207
A nuclear physicist would say that the neutron decays to (proton)+(electron)+(electron anti-neutrino), avoiding any talk of quarks. A particle physicist would say that one of the down quarks within the neutron decays to (up quark)+(electron)+(electron anti-neutrino) and that the resulting three quarks (the decay daughter and the two spectator quarks) are still in a bound state that happens to be a proton. Note that quarks are always in bound states, per quark confinement.
I’m kind of going by intuition here, but it seems to me that it would simply be farther along a continuum. After all, can Pb-178 (with a half-life of 0.23 ms) really considered to be in a bound state?
At the other extreme, while we consider many nuclides to be “stable,” could they not instead simply have a very long half-life? After all, the Standard Model predicts that even a proton has a half life, though estimated to not be less than 10[sup]32[/sup] years.
When I was teaching chemistry, one of the themes that occurred to me about the universe is that it tends to involve continuums rather than discrete categories. While we humans love to categorize things, the universe does not always cooperate.
Some examples: We can define the color “yellow” and “green” on a spectrum, but where does yellow stop and green begin?
Also, we can categorize bonding as covalent or ionic, but what of the bonds that exhibit characteristics of both (e.g. polar covalent bonds, for example)?