Magnesium and aluminum have similar atomic weights, but their chemical properties are quite different. Magnesium is an alkaline earth metal, meaning its outer electrons are less held together than those of aluminum. This makes the magnesium atom slightly less dense and thus lighter. Aluminum atoms, on the other hand, feature a large number of protons that pull more tightly on their outer shells and cause them to be denser. This is why aluminum is heavier than magnesium.