Magnesium (Mg). Atomic weight 24.305. Atomic number 12. Magnesium is a chemical element. It is a shiny grey metal that, when reduced to powder, is highly flammable. In the early years of photography magnesium powder was used as a flash light source. It is the eighth most common metal in the earth's crust and the third most used structural metal because it combines lightness with strength, of great importance in aerospace industries. However, because it is very brittle in its pure form, it can only be used as an alloy. Magnesium compounds can be used in medicines such as laxatives, antacids and to stabilize abnormal nerve excitation or blood vessel spasm in such conditions as eclampsia. When burning it can only be extinguished with a smothering agent like sand or a class D dry chemical fire extinguisher.