Short answers, pulled from the story.
Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal with low density, a low melting point, and high chemical reactivity, and it almost always has an oxidation state of +2. It is two-thirds the density of aluminium and the lowest-melting and lowest-boiling of all the alkaline earth metals.
Magnesium is produced mainly by the Pidgeon process, which calcines dolomite to magnesium oxide and reduces it with silicon at high temperature, and which now dominates world production. The second main route is electrolysis of molten magnesium chloride from brine and seawater, used in the Dow process. World production was about 1,100 kilotonnes in 2017, with China producing 930 kilotonnes.
Magnesium metal was first isolated by Sir Humphry Davy in England in 1808 using electrolysis on a mixture of magnesia and mercuric oxide. Antoine Bussy prepared it in coherent form in 1831. Davy first suggested the name magnium before magnesium became standard.
Burning magnesium reacts violently with water, splitting it to release flammable hydrogen gas that intensifies the fire, so water cannot extinguish it. It also reacts with carbon dioxide to form magnesium oxide and carbon, so carbon dioxide fuels the fire rather than smothering it. Burning magnesium can be quenched with dry sand or a Class D dry chemical fire extinguisher.
Magnesium is the third-most-commonly-used structural metal after iron and aluminium, used chiefly in aluminium alloys, die-casting, sulfur removal in iron and steel production, and titanium production in the Kroll process. It also appears in electronics, fireworks, flares, sacrificial galvanic anodes, and Grignard reagents for organic synthesis.
Magnesium is essential to all cells and is required by more than 300 enzymes, including all enzymes that use or synthesize ATP and those that build DNA and RNA. An adult body contains about 22 grams of magnesium, with 60% in the skeleton, 39% inside cells, and 1% outside them. Good dietary sources include nuts, cereals, cocoa, and green leafy vegetables such as spinach.