Magnesium (Mg). Diagram of the nuclear composition and electron configuration of an atom of magnesium-24 (atomic number: 12), the most common isotope of this element. The nucleus consists of 12 protons (red) and 12 neutrons (yellow). 12 electrons (white) bind to the nucleus, successively occupying available electron shells (rings). The stability of an element's outer electrons determines its chemical and physical properties. Magnesium, vital for cellular function, is an alkaline earth metal in group 2, period 3, and the s-block of the periodic table. In elemental form, it is a reactive metal that burns fiercely when heated in air.