Fission releases a large amount of energy. So does fusion. One change splits the heaviest nuclei, the other merges the lightest, opposite changes in every obvious way. Yet both release energy, and for the same reason.
In the mass defect lesson you saw the fact: in an energy-releasing nuclear reaction, the products together have slightly less mass than the reactants, and that missing mass is the reaction's mass defect. That lesson named the quantity and stopped. This lesson is the why it put off: what the missing mass has to do with the released energy.
Add up the masses of a nuclear reaction's reactants. Then add up the masses of its products.
For an energy-releasing reaction, the products' total is slightly smaller.
Now count the protons and neutrons on each side: the counts balance exactly.
So the missing mass is not a missing particle. Every proton and neutron is still there; the total mass has fallen anyway.
The ledger balances: the mass that left the reaction is exactly the mass equivalent of the energy that left it.
One mechanism covers both changes.
In a uranium-235 fission, the fragments and neutrons together have about 0.1% less mass than the original nucleus plus its absorbed neutron.
In fusion, a helium-4 nucleus made by fusing hydrogen has about 0.7% less mass than the separate nuclei that formed it.
Fission or fusion, the story is the same: a little mass leaves, and the released energy is that mass, converted.
A uranium-235 nucleus plus its absorbed neutron have a certain total mass. After the fission, the fragments and neutrons together have about 0.1% less mass. Where did the missing 0.1% go?
Count the protons and neutrons before and after: the counts balance, so no particle is missing.
The missing 0.1% is mass that the reaction converted.
The products carry that converted mass away as the released energy.
The missing 0.1% of the mass was carried away as the fission's released energy.
A helium-4 nucleus made by fusing hydrogen has about 0.7% less mass than the separate nuclei that formed it. Nikau asks: did the fusion lose protons or neutrons?
Count the protons and neutrons: every one that went in is still in the products.
The 0.7% is missing from the mass total, not from the particle count.
That missing mass left the reaction as the released energy.
No. The fusion kept every proton and neutron; the missing 0.7% of mass left as released energy.
An energy-releasing nuclear reaction runs once. How does the products' total mass compare with the reactants' total mass?
After an energy-releasing nuclear reaction, how does the total count of protons and neutrons compare with the count before?
Blanka says: 'Fission and fusion release energy for the same reason: the products end up with slightly less total mass, and that missing mass leaves the reaction as the released energy.' Is Blanka correct?
Why does a fusion release energy?
In a uranium-235 fission, the fragments and neutrons together have about 0.1% less mass than the original nucleus plus its absorbed neutron. What happened to that missing 0.1%?
Two light nuclei merge, and the new nucleus plus a free neutron together have slightly less mass than the two nuclei had. Which quantity matches the missing mass exactly?