"Magnesium" (symbol Mg) is an alkaline earth metal and has common oxidation number +2. It is the eighth most abundant element in the Earth's crust and the ninth in the known universe as a whole. Magnesium is the fourth most common element in the Earth as a whole (behind iron, oxygen and silicon), making up 13% of the planet's mass and a large fraction of the planet's mantle. It is relatively abundant because it is easily built up in supernova stars by sequential additions of three helium nuclei to carbon (which in turn is made from three helium nuclei). Due to the magnesium ion's high solubility in water, it is the third most abundant element dissolved in seawater.