$$2Mg(s) + O_2(g) → 2MgO(s)$$

This reaction is highly exothermic, meaning that it releases a large amount of heat. It is also a very fast reaction, and can even be explosive if the magnesium is in a fine powder form. The product of this reaction is magnesium oxide, which is a white powder that is used in a variety of applications, such as fire retardants, cement, and fertilizers.