See the answer is that you theoretically could split Aluminum up into lighter elements but it'd cost you a huge amount of energy to do so. Uranium can be split with a net energy gain.
Answered by: Jason Fahrion, B.S., Lab Technician, Portland, Or.
A plot of binding energy for a given atomic weight of the nucleus can be found at http://encarta.msn.com/media_461531006_761558960_-1_1/Nuclear_Binding_Energy.html.
In the case of carbon 12, each individual nucleon (proton or neutron) is bound by 89 MeV / 12, or around 7.45 MeV. As the mass number increases, the binding energy also increases, which means that light elements can release additional energy through fusion. The maximum binding energy per nucleon occurs for Iron 56, and for heavier elements the binding energy decreases as the mass number gets larger. Therefore, a very heavy nucleus such a uranium 238 nucleus can produce additional energy not through fusion, but rather through fission that divides it into two lighter and more tightly bound nuclei. So to answer your question, uranium can naturally undergo fission because fission produces energy, but aluminum is too light to undergo fission. However, at the center of some stars conditions are right for elements such as carbon, oxygen, silicon, etc. to undergo fusion and to continue to produce energy until iron is produced and no more energy can be produced through either fusion or fission.
Finally, I should mention that the binding energy curve is much steeper when increasing for light elements than when decreasing for heavy elements. As a result, while fission releases an immense amount of energy (as evidenced by power plants and atomic weaponry), fusion releases far more energy, which is why researchers are actively working on methods to harness that energy for power production.
Answered by: Charles Steinhardt, B.A., Astronomy Grad Student, Harvard University
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