## Chemistry: The Central Science (13th Edition)

The sphere on the right is the largest, so from left to right the spheres represent: $Mg^{2+}\rightarrow K^+ \rightarrow Cl^-\rightarrow Se^{2-}$
The radius of the atom decreases as the positive charge increases, so the atom with the biggest positive charge has the smallest radius. $Mg^{2+}\lt K^+ \lt Cl^-\lt Se^{2-}$