All the elements here are of the same period with the exception of Ca2+. Ionic radii decreases across a period from Na+ to Al3+ as the number of electrons stay the same while the number of protons increase, thus the protons will pull the electrons closer, causing ionic radii to decrease. There is a big jump in ionic radii from cations to anions across a period, as anions have extra electrons which repel each other, causing ionic radii to increase. Ionic radii for anions decrease across a period, as the number of protons increases. S2- has one less proton than Cl-, thus there is less attraction between the protons and electrons and thus ionic radii is bigger. For Ca2+, the same theory applies. Ca2+ has similar number of electrons have S2-, but more protons, thus attraction is stronger in Ca2+, causing ionic radii to be smaller than S2-.
