atomic structure

R = A^1/3r₀ where r₀ is 1.25 fm,

this is one postulate(maybe) of the Rutherford's model. Read it sumwhere.

here's wiki:
After Rutherford's discovery, scientists started to realize that the atom is not ultimately a single particle, but is made up of far smaller subatomic particles. Following research was done to figure out the exact atomic structure which led to Rutherford’s gold foil experiment. They eventually discovered that atoms have a positively-charged nucleus (with an exact atomic number of charges) in the center, with a radius of about 1.2 x 10-15 meters x [Atomic Mass Number]1/3. Since electrons were found to be even smaller, this meant that the atom consists of mostly empty space.