the covalent radius

When two atoms share electrons rather than trade them, they still sit a definite distance apart, and half of that shared-bond distance is the covalent radius. It answers the same how-big-is-this-atom question as the ionic radius, but for the covalent end of the bonding spectrum, where neither atom has become a charged sphere and the two instead overlap and share a common electron cloud. In diamond, for instance, every carbon-to-carbon distance is about 0.154 nm, so carbon's covalent radius is about 0.077 nm.

The key contrast with ionic radius is what changes the number. An ion's radius swings a lot with its charge and coordination; a covalent radius is more nearly a fixed property of the atom, because no electrons have been fully transferred. Crucially, a covalently bonded atom is usually smaller than the same atom would be as an anion (it has not swollen with extra captured electrons) and larger than it would be as a cation. Silicon's covalent radius (about 0.111 nm) sits between its bulky neutral atom and its tiny Si4+ ion, a reminder that the size of silicon depends entirely on how it is bonded.

For the very covalent ceramics (SiC, Si3N4, boron carbide, diamond, BN), the ionic hard-sphere picture and its radius-ratio predictions simply do not apply, and covalent radii plus bond directions are the right tools. Adding covalent radii of two elements and comparing the sum with the measured bond length is itself a check on how ionic or covalent a real bond is: if the observed spacing matches the covalent sum, the bond is behaving covalently.