Elements that contain unstable nuclei are radioactive; they are called radionuclides. They decay by releasing mostly alpha and beta particles accompanied by gamma rays. An alpha particle has low-energy, is positively charged and consists of two protons and two neutrons, i.e., a helium atom without its 2 electrons; it can be stopped by tissue paper or human skin. A beta particle is a high-energy, negatively charged electron (negatron) or a positively charged positron; it can be stopped by a sheet of aluminum.
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