Iggy's Element Bonanza! Element: Magnesium Atomic Symbol: Mg Atomic Number: 12 Iggy: Magnesium is the second member of the Alkaline Earth Metals and is a shiny, gray solid. It's the third most abundant element dissolved in seawater after Na and Cl. Magnesium is also flammable, especially when in thin strips or powder and burns white. Magnesium has three stable isotopes, Mg-24, Mg-25, and Mg-26.