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covalent radius The atomic radius of an atom (e.g. carbon), determined in a covalent compound by a technique which can be used only for structures in which that atom (carbon) is covalently bonded. Measurement of the distance between planes of atoms (the ‘d-spacing’) gives the sum of the radii (the bond length) of the two covalently bonded atoms in two planes. If all the atoms in the compound are of the same element this gives the value of the radius of an atom in each plane (e.g. the bond length in diamond (pure carbon) is twice the radius of an individual carbon atom). If atoms of different elements are covalently bonded, the bond length of one of the atoms can be derived from the sum of the two different bond lengths only if the bond length of the other atom is known.