begin mathsize 48px style r subscript s m a l l end subscript over r subscript b i g end subscript end style

rsmaller= Radius of the smaller ion (usually the Cation) ; rbigger = Radius of the bigger ion (usually the Anion)    

begin mathsize 48px style space to the power of A subscript P X subscript N space end subscript end style

X = Symbol of the element; A = Mass Number ;  

P = Number of Protons; N= Number of Nutrons