hobz Posted September 28, 2008 Share Posted September 28, 2008 As far as I can tell, an atom's instability is caused by a struggle in the nucleus between the nuclear force and the electromagnetic force. Are neutrons and protons bound by the nuclear force or is it only protons? Iron represents an equilibrium between these two forces. If an atom has more protons (e.g. 92 as is the case with uranium is I'm not mistaken), the electromagnetic force will "win more battles" than the nuclear force, and the atom will be prone to break up into smaller more stable atoms - fission - in which an amount of energy in some form is also released. The energy released is equal to the energy used to bind the original atom together by the nuclear force. If I have a hydrogen atom, and shoot a proton directly into the nucleus, then the two protons fuse together and release some energy. I don't why or how much. I have to shoot my proton rather fast as to get the my proton sufficiently close to the nucleus proton allowing the nuclear force to kick in and overcome the repelling em-force. Are light atoms not also unstable in a way? I mean, could these fusions not occur naturally provided that high energy protons are present. If I fuse iron and a proton, then this configuration will eventually decay into iron again right? Is iron unstable to some degree? Link to comment Share on other sites More sharing options...
swansont Posted September 28, 2008 Share Posted September 28, 2008 As far as I can tell, an atom's instability is caused by a struggle in the nucleus between the nuclear force and the electromagnetic force. Are neutrons and protons bound by the nuclear force or is it only protons? Basically yes, but you have to realize that there are two different nuclear forces — strong and weak. The strong force does act on both neutrons and protons, and nuclei with too many nucleons tends to alpha decay. But an excess of protons or neutrons leads to beta decay, which is mediated by the weak force. Iron represents an equilibrium between these two forces. If an atom has more protons (e.g. 92 as is the case with uranium is I'm not mistaken), the electromagnetic force will "win more battles" than the nuclear force, and the atom will be prone to break up into smaller more stable atoms - fission - in which an amount of energy in some form is also released. The energy released is equal to the energy used to bind the original atom together by the nuclear force. If I have a hydrogen atom, and shoot a proton directly into the nucleus, then the two protons fuse together and release some energy. I don't why or how much. I have to shoot my proton rather fast as to get the my proton sufficiently close to the nucleus proton allowing the nuclear force to kick in and overcome the repelling em-force. Are light atoms not also unstable in a way? I mean, could these fusions not occur naturally provided that high energy protons are present. If I fuse iron and a proton, then this configuration will eventually decay into iron again right? Is iron unstable to some degree? Light atoms can be unstable with beta decay, as I mentioned before. Be-8 is also unstable, splitting into two alphas. Link to comment Share on other sites More sharing options...
Klaynos Posted September 28, 2008 Share Posted September 28, 2008 The Semi-empirical mass formula Which is a good equation for finding binding energies, you find the mass difference between it and the constituent parts and convert that into an energy... Has the following terms: Volume term Surface term Coulomb term Asymmetry term Pairing term The first two are basically the nuclear forces, the coulomb term is clearly the EM force term and the last two are quantum mechanical in nature... Link to comment Share on other sites More sharing options...
hobz Posted September 29, 2008 Author Share Posted September 29, 2008 Light atoms can be unstable with beta decay, as I mentioned before. Be-8 is also unstable, splitting into two alphas. What is the logic behind this? Shouldn't the strong force hold the nucleons together as it does in heavier stable atoms? Link to comment Share on other sites More sharing options...
Klaynos Posted September 29, 2008 Share Posted September 29, 2008 What is the logic behind this?Shouldn't the strong force hold the nucleons together as it does in heavier stable atoms? 2 alphas have a lower energy than Be-8, so it is energetically advantageous to decay. Link to comment Share on other sites More sharing options...
hobz Posted September 29, 2008 Author Share Posted September 29, 2008 So what force overcomes the nuclear force to allow the decay to happen? Link to comment Share on other sites More sharing options...
Klaynos Posted September 29, 2008 Share Posted September 29, 2008 IIRC (I'll check my notes when I get some time on Thursday!) that it can be thought of as tunnelling, so the alpha particle tunnels out of the nucleus, although this description breaks down imo for this type of symmetric breaking... Link to comment Share on other sites More sharing options...
swansont Posted September 29, 2008 Share Posted September 29, 2008 2 alphas have a lower energy than Be-8, so it is energetically advantageous to decay. Right. Alphas are exceedingly tightly bound, which is why they are the particle of choice to be released for nuclei that are too big, but stable vs beta decay. Link to comment Share on other sites More sharing options...
Gilded Posted September 29, 2008 Share Posted September 29, 2008 If I fuse iron and a proton, then this configuration will eventually decay into iron again right? Is iron unstable to some degree? Actually if you add a proton to Fe-58 (which is a rare but stable isotope of iron) you'll get Co-59 which is stable. But if you add a neutron you get Fe-59 which is actually quite unstable with a half-life of about 44.5 days. This is a fine example of this balance being quite easily upset. "Some degree" is a pretty ambiguous term in nuclear physics. For example one might say bismuth is unstable to some degree as it has a half-life of ~1.9*10^19 years. 1 Link to comment Share on other sites More sharing options...
hobz Posted September 30, 2008 Author Share Posted September 30, 2008 "Some degree" is a pretty ambiguous term in nuclear physics. For example one might say bismuth is unstable to some degree as it has a half-life of ~1.9*10^19 years. Exactly what I meant by "some degree" thank you. So "tunnelling" is an event that, when it occurs, can escape the nuclear force? Link to comment Share on other sites More sharing options...
Klaynos Posted September 30, 2008 Share Posted September 30, 2008 Are you familiar with quantum mechanical tunnelling of electrons in potentials? This is the same physical process but with Alpha groups of particles and the potential is due to the binding force of the nucleus. Link to comment Share on other sites More sharing options...
hobz Posted September 30, 2008 Author Share Posted September 30, 2008 No I am not familiar. Just did a read up on wikipedia. Interesting stuff. I am guessing that the probability a decay occuring is related to the square of the wavefunction for a given atom. Small probabilities of tunnelling occuring, give a high stability (long half-life). Are all atoms subject to this tunneling effect? If they are, then that would mean that all atoms are unstable. 1 Link to comment Share on other sites More sharing options...
swansont Posted September 30, 2008 Share Posted September 30, 2008 Are all atoms subject to this tunneling effect? If they are, then that would mean that all atoms are unstable. The system has to have a lower energy after undergoing decay for the reaction to be possible. Not all atoms can do this. Link to comment Share on other sites More sharing options...
Tsadi Posted September 30, 2008 Share Posted September 30, 2008 No I am not familiar. Just did a read up on wikipedia. Interesting stuff. I am guessing that the probability a decay occuring is related to the square of the wavefunction for a given atom. Small probabilities of tunnelling occuring, give a high stability (long half-life). Are all atoms subject to this tunneling effect? If they are, then that would mean that all atoms are unstable. Yes. What your referring to is called the probability density. [math]\int_{\Omega} |\psi \psi*|^{2}=1[/math] It basically measures a collapse in the wave function. Link to comment Share on other sites More sharing options...
swansont Posted September 30, 2008 Share Posted September 30, 2008 Yes. What your referring to is called the probability density. [math]\int_{\Omega} |\psi \psi*|^{2}=1[/math] It basically measures a collapse in the wave function. How is this relevant to the question? Link to comment Share on other sites More sharing options...
Tsadi Posted September 30, 2008 Share Posted September 30, 2008 He said he was guessing. I just sounded his arguement. What is wrong with that? Link to comment Share on other sites More sharing options...
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