The atomic radius of an element is difficult to precisely define because the size of the electron cloud is uncertain. There are several definitions used, including half the shortest distance between nuclei in an element's structure (covalent radius for non-metals), half the distance between two identical atoms covalently bonded (covalent radius), and half the shortest distance between non-bonded atoms (van der Waals radius). The metallic radius is also used, which is half the shortest distance between nuclei in a metal.