In a nuclear fission reaction, the sum of atomic numbers (proton numbers) and the sum of atomic masses (mass numbers) are conserved, meaning they remain the same before and after the reaction. This principle is known as the conservation of mass and charge.
However, when it comes to the masses on both sides of the reaction, we say that they are not equal due to the conversion of mass into energy according to Einstein's famous equation, E=mc². This equation states that mass (m) can be converted into energy (E) and vice versa, where c is the speed of light.
During a nuclear fission reaction, a heavy nucleus typically splits into two or more lighter nuclei. The total mass of the products (the lighter nuclei) is generally slightly less than the mass of the original nucleus. The missing mass is converted into energy according to Einstein's equation. This released energy is often in the form of kinetic energy of the fission products, as well as in the form of gamma radiation and other particles.
Therefore, although the sum of atomic numbers and atomic masses remains conserved in a fission reaction, we observe that the masses on both sides are not equal due to the conversion of mass into energy. This is in accordance with the principle of mass-energy equivalence.