One way to think about atomic size is to look at the units that form when atoms of the same element joined to one another. These units are called molecules. Because the atoms in each molecule are identical,The distance between the nuclei of these atoms can be used to estimate the size of the atoms.The size is expressed by the atomic radius!!! What is that you may ask. Atomic Radius is one half of the distance between the nuclei of two atoms of the same element when the atoms are joined.