Physics Van 3-site Navigational Menu

Physics Van Navigational Menu

Q & A: fission energy

Learn more physics!

Most recent answer: 10/22/2007
when nuclear fission happens, why is there energy released? isn’t the total mass of the fragments larger than the original nucleus, meaning that energy has converted into mass?
- Murk (age 18)

You've got the idea right but guessed the fact wrong. The sum of the (rest) masses of the fragments is SMALLER than the original mass, so that leaves some energy for other forms.

Although the rest mass per nucleon of nuclei tends to go down as the nucleon number goes up for the light nuclei (for which fusion relases energy), it goes UP as the nucleon number goes up for heavier nuclei. I belive that the minimum energy per nucleon occurs for one of the isotopes of iron.

Mike W.

The reason why the rest mass of a nucleus divided by the number of protons and neutrons isn't constant is because of the binding energy that sticks them together. Protons and neutrons hold together inside a nucleus with the strong nuclear force, and the total energy of the stuck-together combination is much less than having all the protons and neutrons separated (it takes energy to pull nuclei apart because they are held together so tightly). Because energy and mass are different names for the same thing, the fact that a nucleus has less energy than its pieces when separated, means that it has less mass. The odd thing, is that this difference in mass between the nucleus and the sum of its parts depends on how many protons and neutrons there are in the nucleus. While the most stable combination is an isotope of iron, there are many heavier elements to be found in the Earth's crust where we can dig them up. The best model explaining their existence is that these heavy elements were formed in a supernova explosion billions of years ago.


(published on 10/22/2007)

Follow-up on this answer.