Well, if the ion has a greater radius than the potassium, the reason that ion cannot pass across is simply because it's too large. Its diameter is too large, and it cannot actually move across this three angstrom width. Now, what happens if the ion is smaller than potassium? That's the slightly more difficult a question to actually answer. So, for instance, let's compare sodium and potassium. Now, Potassium Dionic radius of potassium is about 1.33 angstromes, while the ionic radius of sodium is about 0.95.