Question

calculate energy released in Nuclear Fission in terms of a specific binding energy

Answer 1

Consider the neutron induced fission U-235+n→⋯→La-139+Mo-95+2nU-235+n→⋯→La-139+Mo-95+2n, where …… denotes intermediate decay steps.

I want to calculate the released energy from this fission. One way would be to calculate the difference of the binding-energies (BB):

ΔE=B(139,57)+B(95,42)−B(235,92)≈202,3MeVΔE=B(139,57)+B(95,42)−B(235,92)≈202,3MeV

(Btw. I didn't use the binding energies from a semi-empirical binding energy formula but calculated them directly via mass defect).

Another way is:

ΔE=(m(U-235)+mNeutron−m(Mo-95)−m(La-139)−2mNeutron)c2≈211,3MeVΔE=(m(U-235)+mNeutron−m(Mo-95)−m(La-139)−2mNeutron)c2≈211,3MeV

Which one gives the correct result? Why?

You notice that 57+42≠9257+42≠92, if that was the case, it would be equal, but I don't clearly see where the difference physically comes from and what to add or subtract (and why) from to the first or from the second term to get the other result. How to make this clear?

A slightly other point of view: What different questions do both calculations answer?