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I think I am confused between the mass defect of just splitting up a nucleus into its component nucleons and the mass defect involved between the two sides of a nuclear transmutation reaction. I assume the problem refers to the second, and the answer is B by just plugging in the numbers of the table. However can somehow please explain why the mass of the left side of the reaction is less than the right? I thought when you SPLIT up an atom (as in the Lithium to 2 Heliums) you have to put in energy therefore the mass-energy should be higher in the product side?

Thanks in advance!


1 Answer 1


It may take energy to start the reaction, or in the case of nuclear reactions, to get the nuclei close enough to react. However, the process of the reaction may release more energy than it took to start the reaction. That is the goal in nuclear power -- to release more energy than it takes to start the reaction.

To correct your statement a little, the mass on the left is GREATER than the mass on the right. The mass-energy will be the same on each side, as any mass lost will be converted to energy. The equation shown does not show the energy that goes in or out of the system, but the change in mass will allow you to calculate the net energy gained or lost. Since E=mc^2, the loss of mass going from left to right will indicate an increase (release) of energy from left to right.

  • $\begingroup$ Could you just explain more clearly why the mass on the left is greater than the mass on the right? Thanks! $\endgroup$ Aug 30, 2013 at 7:05
  • $\begingroup$ Your question could be answered in a couple of ways. One way is just what you did in solving the problem -- you add the masses on each side and compare, and the left side has more mass. $\endgroup$
    – jdj081
    Aug 30, 2013 at 12:28
  • $\begingroup$ ...If you want to know WHY it is that way, it is related to the particular reaction. Reactions either gain or lose energy (exothermic vs. endothermic). This is an example of a nuclear reaction that gains energy, and to do so it loses mass. The increase in energy is equal to the decrease in mass times c^2. $\endgroup$
    – jdj081
    Aug 30, 2013 at 12:36

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