Since the radioactive isotopes are unstable, their nuclei disintegrate and the process is called nuclear decay. If after the nuclear decay the resulting products are not stable, another nuclear decay will occur and this process will be repeated until a stable product is released. For example, the uranium isotope U-238 will pass through 14 nuclear decay sequences until the stable product Pb-206 is released and the decay process will stop.
The ratio of protons to neutrons within an atom decides the radioactivity levels of an atom. If this ratio falls outside a fixed interval of stability, then the isotopes of an element become radioactive.
The three types of radioactive decay are: alpha particle emission, beta particle emission, and gamma radiation.
Alpha particles from alpha particle emissions contain 2 protons and 2 neutrons and are positively charged particles of helium nuclei. The alpha particle has no electrons and since the electrons can be gained easily, the alpha particle can gain the needed 2 electrons and the helium atom becomes neutral.
Uranium and thorium lose 4 units of mass at each emission of an alpha particle.
Alpha particles are emitted by Uranium, Radium,Thorium and elements larger than Uranium. The emissions of alpha particles are dangerous if they occur inside the human body.
Beta particles from beta particle emissions are an electron emitted by the nucleus. The beta particles can penetrate human skin more easily than alpha particles but they also can be stopped by aluminum shields. Gamma radiation is more dangerous than beta emissions because they can deeply penetrate human tissue.
Since gamma emission does not involve a change in mass it can be considered as gamma radiation. Usually, gamma emission coincides with alpha and beta emissions.