Question

In a nuclear reaction, the mass of the original particles does not equal the mass of the particles that form during a reaction, WHY?

Answer 1

In a nuclear reaction, the identity of the atomic nucleus changes, therefore, after the reaction, the mass will be different.

Answer 2

When energy is released or absorbed into a reaction there is a change in mass according to Einstein's equation E=mc^2. But it is usually small so it is not calculated except in nuclear reactions. So it can be said (kind of) that mass turns into massless energy. You can go onto Yahoo for more explanation