What is conserved during radioactive decay?

During radioactive decay, the quantity that is conserved is mass, although this requires a nuanced understanding in the context of relativistic physics. In the context of classical physics, mass may appear to be lost in processes like beta decay, where a neutron decays into a proton while emitting a beta particle and an antineutrino. However, in terms of energy-mass equivalence, the total mass-energy is conserved, as the mass lost in the form of particles is converted to energy according to Einstein's mass-energy equivalence principle (E=mc²).

The process of decay ensures that while the identity of the nucleus changes and particles are emitted, the overall conservation laws hold true when one accounts for both mass and energy. Therefore, mass conservation can be understood in terms of the totality of the system, emphasizing the conservation of total energy, which is frequently discussed in addition to the changes in mass associated with nuclear reactions. In the framework of nuclear physics, one can also express this conservation alongside the concept of atomic number and atomic mass units, but fundamentally, mass is recognized as conserved when balanced against energy changes in nuclear processes.