What happened in fission when the products have less mass than the starting material?

Answer 1

The binding energy per nucleon varies in different nuclei, being a maximum in the region of iron and nickel, and getting progressively less as the heavier nuclei are approached, Therefore when a uranium nucleus splits into two nuclei of lighter elements, the total binding energy is increased, and this results in a loss of mass. The destroyed mass appears as energy, from the relation E = mc2. You can read more and see the binding energy graph in the link below. It is also evident why fusion of light nuclei like hydrogen also releases energy, as in this part of the graph binding energy increases as the nuclei get heavier.

Answer 2

The mass deficit is caused by the fact that the binding energy per nucleon increases as the size of the nucleus reduces, until iron/nickel are reached when it is a maximum. The equivalent energy to the mass deficit, from E = mc2, is released as energy. See the link below for more information and a graph of nucleon binding energy (a nucleon is another name for a proton or a neutron)

Answer 3

When the products of a fission reaction have less mass than the starting material, the "missing" mass has been converted into energy following Einstein's famous equation, E=mc^2. This energy is released in the form of gamma rays, kinetic energy of the products, and neutrons.