I am aware that n is the principal quantum number and determines the energy of a specific energy level of an atom. In my notes, I see that n goes from 1,2,3... which implies to me all the way to infinity. If this is the case, why doesn't this imply that there can be infinitely many shells in an atom and consequently make the atom infinitely big? I have also read about there being a 'series limit' and know that the atom is about 10^-10m big, so there must be a 'cut off' number of shells somewhere? Many thanks.