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The atomic radius of a chemical element is a measure of the size of its atom, usually the mean or typical distance from the center of the nucleus to the outermost isolated electron. Since the boundary is not a well-defined physical entity, there are various non-equivalent definitions of atomic radius. Five widely used definitions of atomic radius are covalent radius, Van der Waals radius, charge radius, ionic radius, and metallic radius. Typically, because of the difficulty to isolate atoms in order to measure their radii separately, the atomic radius is measured in a chemically bonded state; however, theoretical calculations are simpler when considering atoms in isolation. The dependencies on environment, probe, and state lead to a multiplicity of definitions.