The mass number and atomic number are unchanged. Neutron emission from the decaying nucleus, results in a one unit decrease in the mass number, while the atomic number stays the same. When a large amount of decay energy is available, beta-delayed emission of neutrons, protons, and other particles may occur. Spontaneous fission is the splitting of the nucleus into rougly equal parts, forming two new isotopes of approximately half the parent isotope's mass. In addition multiple neutrons, protons or alpha particles may be emitted at the same time.
Energy is released in the process of particle decays.
Slowing particles will emit gamma, x-ray or even visible light energy. An example is the eerie blue Cherenkov radiation glow seen around water cooled reactors. Gamma radiation may take away some of this mass as well during internal re-arrangements in the nucleus, as in isomer transitions. Gamma rays penetrate further than all particle radiations, simply because they are not slowed by collisions with other atoms within a substance. Gamma rays are produced during nuclear transitions, whereas x-rays are generally produced by inner electron transitions.