Sodium and potassium ion channels have several negatively charged residues at the entry to the channel.
Identify the bases on which $K^+$ channels specifically select for $K^+$ ions, in other words, why do $K^+$ channels not enable $Na^+$ ions to cross the membrane?
ionic radius, $Na^+$ is too big
ionic radius, $Na^+$ is too small
charge, Na is slightly more electronegative than K and does not interact with the channel carbonyl groups
charge, Na is slightly less electronegative than K and does not interact with the channel carbonyl groups
energy cost, it is too energetically costly to dehydrate $Na^+$
