The molecule must be small enough to pass through the membranous pores and it must not have water in it as the lipid tails of the cell membrane are hydrophobic
There are a few vital characteristics to mention here. First we have to know about which ions we're talking about as the cellular membrane is semi-permeable (lets some flow freely through the membrane and some not), it depends on the size of the compounds, and also the charge that a molecule has.
