The covalent radius of an element is considered to be one half of the covalent bond distance of a molecule where the atoms concerned are participating in single bonding. In other words, It may be defined as one-half of the distance between the centers of the nuclei of two similar atoms bonded by a single covalent bond.
The covalent radius of the elements is shorter than its van der Waals radius. The formation of a covalent bond involves the overlapping of atomic orbitals. As a result of this, the internuclear distances between the covalently bonded atoms is less than the internuclear distances between the non-bonded atoms.
The covalent radii vary with the type of the bond between the atoms. The values of covalent radii may also differ between allotropes. For example, the covalent radius of carbon is 77pm in diamond and 71pm in graphite.