Radionuclides, such as radium and uranium, primarily emit alpha particles. Natural radioactivity is the property of some naturally occurring, usually heavy elements, that are heavier than lead. Naturally-occurring radioactive material (NORM) exists in nature and no additional energy is necessary to place them in an unstable state. In general there are two classifications of radioactivity and radionuclides: natural and artificial (man-made). The first radionuclide in the chain is called the parent the subsequent products of the transformation are called progeny, daughters, or decay products. This series of transformations is called the decay chain of the radionuclide. Transformation or decay results in the formation of new nuclides some of which may themselves be radionuclides, while others are stable nuclides. These unstable atoms are called radionuclides their emissions are called ionizing radiation and the whole property is called radioactivity. Each type of decay is typically accompanied by the emission of gamma rays. Unstable nuclides undergo radioactive transformation, a process in which a neutron or proton converts into the other and a beta particle is emitted, or else an alpha particle is emitted. The numerical combination of protons and neutrons in most nuclides is such that the nucleus is quantum mechanically stable and the atom is said to be stable, i.e., not radioactive however, if there are too few or too many neutrons, the nucleus is unstable and the atom is said to be radioactive. Atoms with different atomic mass but the same atomic numbers are referred to as isotopes of an element. All atoms of an element have the same number of protons (this is given by the atomic number) but may have different numbers of neutrons (this is reflected by the atomic mass numbers or atomic weight of the element). The nuclide is characterized by the composition of its nucleus and hence by the number of protons and neutrons in the nucleus. Nuclide is the general term referring to any nucleus along with its orbital electrons. Atoms in turn are made up of neutrons, protons and electrons: neutrons and protons in the nucleus and electrons in a cloud of orbits around the nucleus. The substances we call elements are composed of atoms.
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