In alpha decay, the parent atom emits an alpha particle and results in a daughter nuclide . Immediately following the alpha particle emission, the daughter atom still has the Z electrons of the parent – hence the daughter atom has two electrons too many and should be denoted by . These extra electrons are lost soon after the alpha particle emission leaving the daughter atom electrically neutral. In addition, the alpha particle will slow down and lose its kinetic energy. At low energies the alpha particle will acquire two electrons to become a neutral helium atom. The alpha decay process is described by:
The process of alpha decay is found mainly in proton rich, high atomic number nuclides due to the fact that electrostatic repulsive forces increase more rapidly in heavy nuclides than the cohesive nuclear force. In addition, the emitted particle must have sufficient energy to overcome the potential barrier in the nucleus. The height of the potential barrier is about 25 MeV. Nevertheless, alpha particles can escape this barrier by the process of quantum tunnelling.
J. Magill and J. Galy, Radioactivity Radionuclides Radiation Springer Verlag, 2005