Acids are donors of protons (H+) and bases are acceptors of protons. For example: 1) hydrochloric acid (HCl) in reaction with water give one proton to water and become chloride anion (Cl-). 2) ammonia (NH3) Is base, in reaction with water accepts one protone and become ammonium cation (NH4+).
As the charge of all electrons are equal, the repulsive force exerted by each of them is also going to be equal. So, as K has more electrons repulsing its valence electron than Na, it has greater electron shielding.
