Magnesium. Strips of the reactive metallic element magnesium (Mg). The strip at left is tarnished and appears darker. This is due to the formation of magnesium oxide (MgO) on its surface as the metal reacts with the oxygen and water in the air. The strip at right has been cleaned to reveal the shiny surface of the pure metal. If left exposed to the air, this strip will tarnish and appear like the other strip. The reactivity of magnesium leads to its use as a sacrificial metal to prevent other metals from corroding. It is a light metal with major uses in lightweight alloys and castings. It is mostly obtained from sea water.