Any elements can be fused;
if the resultant element is lighter than Iron (Fe), the resultant atomic mass is less than the sum of the two original masses, the difference being converted to energy, according to the equation E = mc².
Fusion of light elements; energy released
If the resultant element is heavier than Fe, the resultant atomic mass is greater than the sum of the two original masses, the difference being converted from energy, according to the equation E = mc².
Fusion of heavy elements; energy absorbed
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Any elements can be “split”;
if the original element is heavier than Fe, the resultant sum total atomic mass is less than the original mass, the difference being converted to energy, according to the equation E = mc².
Fission of heavy elements; energy released
If the original element is lighter than Fe, the resultant sum total atomic mass is greater than the original mass, the difference being converted from energy, according to the equation E = mc².
Fission of light elements; energy absorbed
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See the reciprocal nature of the relationship? The mass defect is a “store” of energy, and it has a symmetry around Fe. I guess that’s why some stars fizzle out into brown dwarfs…
Hence atomic fission generally involves heavy elements such as Uranium & Plutonium, while fusion involves light elements such as Hydrogen & Helium.