The atomic radius of carbon atom is 70 picometers. The picometer is a very small unit of length, and 1 picometer = 1 x 10-12 meters. The atomic radius is the distance between the nucleus of the atom and the electrons in the outermost shell of the atom.
In the case of the carbon atom, the calculated atomic radius is 67 picometers. The empirical value of the atomic radius is based on observations.
The atomic radius changes slightly when the atom is in an excited or high-energy state. The length of ionic and covalent bonds is used for calculating the atomic radius of various atoms. The calculated and empirical atomic radii may differ slightly.