#### Answer

Though an exact outer boundary cannot exist for an atom according to the quantum theory model, atomic radii can still be defined in several ways, based on the distance between atoms in several situations.

#### Work Step by Step

Though an exact outer boundary cannot exist for an atom according to the quantum theory model, atomic radii can still be defined in several ways, based on the distance between atoms in several situations.
1) If 2 atoms collide then they will ricochet apart due to the electron repulsions.
Therefore, the shortest distance that 2 atoms can get to each other can be assigned to be twice the radius of each atom. That is called the nonbonding atomic radius.
2) In another situation, 2 atoms are bonded to each other in a compound. We can define a bonding atomic radius to an atom as half the distance between the nuclei of the 2 atoms in a bonding.