In terms of size, between which of the spheres would you find the ca2+ and s2− ions?

Ca2+ ions are smaller than S2- ions. This is due to the reasoning that less electrons = smaller atoms. Since electrons are constantly repelling each other in the electron cloud, more electrons allows for a larger shell. However, if the number of electrons are equal, then the ion with more protons will be smaller. This is due to the protons pulling the electrons closer to the center resulting in a smaller sphere.

The answer would be