The atomic radius of a chemical element is a measure of the size of its atoms, usually the mean or typical distance from the center of the nucleus to the boundary of the surrounding shells of electrons. Since the boundary is not a well-defined physical entity, there are various non-equivalent definitions of atomic radius. Four widely used definitions of atomic radius are: Van der Waals radius, ionic radius, metallic radius and covalent radius. Because atom can't be isolated to measure it's radius separately, atomic radius is measured in bonded state of atom. Now, because atom binds differently depending on the type of element an atom is,we have various type of definitions.