The atomic radius of chlorine is approximately 79 picometers. One picometer is equal to a trillionth of a meter, commonly written as 10-12 meters. For example, 79 picometers is equal to 79*10-12 meters.
The atomic radius of an atom is the averaged distance from the center of an atom to the outer boundary of the orbitals that electrons occupy. Atoms are not perfect circles, which is why the actual radius may be larger or smaller at any single point. While most of the mass is concentrated in the nucleus of the atom, most of the volume is occupied by the electron clouds. Because of this, the atomic radius is many times larger than the radius of the nucleus.