- Nuclear fusion occurs when two light nuclei fuse together into a heavier nucleus
- Nuclear fission occurs when a heavy, unstable nucleus breaks apart into two or more lighter nuclei
In both processes, the mass of the products is always smaller than the mass of the initial nuclei. This means that part of the initial mass has been converted into something else: into energy, which is released in the process.
The amount of energy released in the process can be calculated by using the famous Einstein's equivalence:
where m is the difference between the mass of the product and the initial mass of the nuclei, and c is the speed of light.
The answer is energy
The answer is; mass-energy
Radioactive elements are heavier unstable isotopes of the element that sponteneously decay into other elements. In nuclear fission, the isotopes decay into light more stable elements. The change in mass is because some of the matter is converted to electromagnetic radiations. This is why nuclear fission produces harmful radiation such as gamma rays that can be harmful to genetic material of living organisms.