I don't understand this about fission and fusion, could somone help me?

My book says: "We have a fission when a heavy nuclear core is split into two lighter fragment cores. We have a merger when two light nuclear cores join forces into a heavier nuclear core."

My teacher says, that in both fission and fusion we experience mass loss. Therfore the starting product weighs more than the final product if we put it on a weight. But my book says that in fusion the nuclear core will be heavier.

3 Answers
Feb 12, 2018

See explanation -


You do not specify which nuclear cores are being "fused". Not all choices of nuclei to fuse will release energy.

A fusion event such as those that happen in the sun (or within a thermonuclear device) creates heavier nuclei from fused hydrogen (or hydrogen isotopes), and releases energy. The resulting product is a nucleus that is heavier than either of the hydrogen nuclei that produced it, but just very slightly LESS than the sum of the original two nuclei. The "missing" mass is converted to energy at an amount given by Einstein's famous #e = mc^2#.


Feb 12, 2018

#E = mc^2#
Einsteins theory of relativity says that matter is simply a concentrated form of energy


In a fusion reaction, two lighter nuclei combine to form a heavier element: This is the reaction occurring in stars like our sun. However, if you add up the masses of a H-1 and H-3 to create He-4, you would find that the Helium nucleus weighs slightly less than the sum of the masses of the hydrogen nuclei. The difference is called the mass defect and it is the binding energy of the new nucleus, the energy that holds it together.

In a fission reaction, a large nucleus breaks apart into several smaller nuclei, but again the sum of the parts would be slightly less than the original mass. The missing mass (mass defect) is released as energy according to the famous equation above.

Feb 12, 2018

"I don't understand how fusion form a heavier element when for example He-4 weighs less then the sum of the masses of the hydrogen nucleus. Because then He-4 is not heavier then the hydrogen nucleus."!


Perhaps this will help then...

By Sarang - Own work, Public Domain