What effect does an additional proton have on atomic size?
The extra proton in the atom’s nucleus pulls all the inner electrons closer. The outer electrons are screened by the inner electrons, but in general they too are pulled in closer, because the inner electrons can’t hide all of the additional +1 charge from those in the outer shell. So an additional proton will make the atom smaller. This is rather like dropping a queen bee into the center of a swarm. The bees are attracted to the queen, and they close in to attend her. The size of the swarm goes down, even though you’ve increased the mass of the swarm.
