Atomic radius can be determined for an atom that is NOT bonded to another element, or for a bonded atom. How close can a single atom approach another atom before colliding and bouncing off? This is the nonbonding radius, or van der Waal's radius.
When an atom bonds to another atom, there is an attractive force drawing the atoms closer to each other than they normally would be. This is called the bonding radius, or covalent radius. This radius will be slightly shorter than the nonbonding radius.
Source: illustrated by Heesung Shim via UC Davis Chemwiki
