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-.
SO HOPE U UNDERSTAND THIS.