Missing Mass

Where did the mass go?

Still, the question remains, if a uranium atom decays into thorium and an alpha particle, where did the missing 0.0046 AMU go?

As Einstein said,

There was a lot of energy stored up in the residual strong force that held the uranium atom together. When that uranium atom underwent radioactive decay, some of this energy was released as kinetic energy (the energy of motion). This conversion of energy was recorded as a loss of mass.