Magnesium oxide, MgO, has the same type of structure as NaCl (i.e. the anions form a face-centred cubic structure and the cations occupy octahedral sites in the lattice of anions). The density of magnesium oxide is 3.58 g cm?3. Given that the radius of the O2- ion is about 140 pm in most oxides, calculate the radius of Mg2+, in pm. Your solution should include a sketch one face of the unit cell of MgO to show how the edge length of the unit cell is related to the ionic radii