There’s more to it than their atomic weights. Magnesium (24 amu) is ~66% the density of aluminum (27 amu). As I understand it, the crystalline structure of alum

There’s more to it than their atomic weights. Magnesium (24 amu) is ~66% the density of aluminum (27 amu). As I understand it, the crystalline structure of aluminum is more “tightly packed” than that of magnesium.