Explain Nuclear Fusion & Fission Reactions

[frozen7]

How should I explain the particle which has the lower nucleon number will undergo fusion reaction while the partivle which has the higher nucleon will undergo fission reaction?
Thanks.

 

[HallsofIvy]

The question, as phrased, makes no sense. It seems to imply that, given any two atoms (not particles: you can't have an atomic reaction with just a proton!) the one with the lower nucleon number will "undergo fusion reaction" and the other will "undergo fission reaction". In fact, it sounds like you are saying that there is some kind of interaction between the two particles that causes one to "fusion" while the other "fissions"!

What you need to look at is the mass "excess" of the individual nuclei. The mass of a given nucleus is not the same as the sum of the masses of the protons and neutrons making it up- the "binding energy" contributes mass. Further, the total mass of a He necleus (made up of two protons and two neutrons) is less than that of two Deuterium neuclei (made up of one proton and one neutron) (the "excess" of mass of the necleus above that of its constituent parts decreases) so that if you can force two Deuterium nuclei to fuse to form one Helium nucleus, some of the mass is converted to energy. The "excess" decreases down to, I think, lead which has the smallest excess of mass. Then it starts back up so that combining two smaller nuclei to make a larger one requires a lot of energy rather than releasing it. On the other hand, breaking a larger nucleus into two smaller ones winds up with less mass than you started with- the extra being converted to energy- fission.
Uranium has a large enough excess that if it breaks into two smaller nuclei you get out enough energy to sustain a reaction.

 

[lightgrav]

Natural processes occur in a direction which lowers the Potential Energy.
(masses fall in a gravity field, electrons find their way to +V ...)

The "mass per nucleon" as a function of nucleon number looks kind of like:
|\
|.|
|.|
|..\
|. .|
| . . \ . . . . . . . . . . _ . . . -
| . . . . \ . . . . _ . -
| . . . . . - -
----------Fe----------------

which is a graph of Potential Energy as a function of nucleon number (mass #?)
Given enough time and enough jostling, every nucleus will become iron.

Your textbook might have a picture of this graph "upside down", called
the "Binding Energy" curve.