Atomic radius of an atom cannot be measured directly. The reason is that atoms are not found in isolation or a single atom is always in unstable form and it wants to make bonds with the other atoms. There are electrons which are present in the outer most shells of the atoms. Therefore, when an atom reacts with the other atom then it can gain electrons (which increases its size of shell) or loses its electrons (decreases the radius of the shell). The atomic radius of the atom is usually considered as the distance of nucleus from the outer most shell and hence if an electron reacts then the atomic radius can be changed therefore, atomic radius is considered as the half of the distance between the nuclei of two atoms forming bond with each other.