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Physics, Astronomy & Cosmology / Re: Does a fissionable nucleus weigh less than the sum of its components?
« on: 21/09/2017 21:56:33 »Thanks kryptid .Could you clarify it a bit please, as by the calculations you did add up but it doesnt get me any closer to why. The mass defect , lower potential energy means lower overall mass, so that must mean that the split atom weighs more ? I know the sums check out, but i was just unable to find why.
Yes, a uranium-235 nucleus (plus a neutron) weighs more than the resulting krypton-92 and barium-141 nuclei (plus 3 neutrons) added together. Different isotopes have different stability, which means different levels of binding energy per nucleon.
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The products usually weigh more than the original atom due to nuclear binding energy
This is only true if the isotopes you are trying to break apart are very lightweight.
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how does it differ in fissile materials.
The interesting thing about that is that you don't always get a net energy release by splitting an atom. It is highly dependent upon the isotope being used. As a general rule of thumb, lightweight nuclei (those that weigh less than iron) will release energy when they are fused together and absorb energy when they are broken apart. The opposite is true of isotopes that are heavier than iron. This is due to differences in the nuclear binding energy per nucleon. Iron-56 has the lowest mass per nucleon of all known isotopes, whereas nickel-62 has the highest binding energy of any known isotope. Fissile materials like uranium-235 are much, much heavier than iron or nickel isotopes, and as such can release energy when they are broken apart. What makes uranium-235 fissile is its ability to sustain a chain reaction by releasing neutrons of the right energy when it undergoes fission. Not all isotopes of uranium can do that. Uranium-238 can undergo fission if it is struck by a fast neutron, but it does not release more fast neutrons in the process to continue the reaction: any neutrons released are so slow that they are very unlikely to be absorbed by another nucleus.
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The constituents of a carbon 12 weigh more than the carbon twelve atom, so removing a neutron means you have to add energy.
The reason is because the neutron is strongly bound within the nucleus, so some minimum activation energy is needed to liberate the neutron from the strong nuclear force's grip. In the case of removing a neutron from carbon-12 to create carbon-11:
12C → 11C + n
(12 amu) - (11.0114336 amu + 1.00866491588 amu)
= 12 amu - 12.02009851588 amu)
= - 0.02009851588 amu
In the case of this reaction, the products are less stable than the reactant. That's why this reaction requires net input energy while the fission of uranium-235 has a net release of energy instead.
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Fissionable materials like uranium 238 will not under go splitting and have to undergo processes to be turned into fissile materials, so how does fission material uranium 235 end up weighing less than its constituent parts ? Uranium I know is unstable and the atom decays naturally, qnd this obviously has something to do with it, but I would have thought a 238 was more ready to loose a neutron than the 235 but apparently not.
As I've already stated, uranium-235 weighs less than its constituent particles because nuclear binding energy makes it more stable than all of those nucleons would be if they were free. It's actually rather like the difference between a rock being held in your hand and that same rock on the ground. The rock on the ground is in a lower gravitational potentially energy state than when it is in your hand. The rock in your hand actually weighs a tiny bit more than when it is on the ground because it's in a higher potential energy state. Bizarre, but true.