Chemistry. Chemical element with the atomic symbol Mg. A light, silvery, metallic element belonging to the alkaline earth metals. It is an essential mineral for humans, animals and plants. It is required for the activity of many enzymes, especially those concerned with ATP. Most of it is maintained in bones and teeth. It plays an important role in information transmission on cellular level and neuromuscular transmission. The other essential role of magnesium is as the central ion of chlorophyll.