# Which of the following atoms will have the largest and the smallest size?

## $M g$ $M {g}^{+ 2}$ $A l$ $A {l}^{+ 3}$

So, with respect to the atoms, aluminum ($Z = 13$) is SMALLER then magnesium ($Z = 12$).
With respect to the ions, both $A {l}^{3 +}$ and $M {g}^{2 +}$ have a Noble Gas configuration (cf. $N e$, $Z = 10$). Because aluminum ion is tripositive, its ionic radius should be smaller than that of $M {g}^{2 +}$, because the excess nuclear charge should compress the valence radii.
So we've made a prediction. Our job is not finished as physical scientists; we need data on the ionic radii. You will find this on the web; suitable units would be $\text{pm}$, $\text{1 pm } = {10}^{-} 12 m$.