Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.


Magnesium, symbol Mg, silvery white metallic element that is relatively unreactive. In group 2 (or IIa) of the periodic table (see Periodic Law), magnesium is one of the alkaline earth metals. The atomic number of magnesium is 12.


The metal, first isolated by the British chemist Sir Humphry Davy in 1808, is obtained today chiefly by electrolysis of fused magnesium chloride. Magnesium is malleable and ductile when heated. With the exception of beryllium, it is the lightest metal that remains stable under ordinary conditions. The metal is not attacked by oxygen, water, or alkalies at room temperature; it reacts with acids. When heated to about 800° C (about 1472° F), it reacts with oxygen and emits a brilliant white light. Magnesium melts at about 649° C (1200° F), boils at about 1107° C (about 2025° F), and has a specific gravity of 1.74; the atomic weight of magnesium is 24.305.

Magnesium ranks sixth in natural abundance among elements in crustal rocks. It occurs in nature only in chemical combination with other elements, particularly as the minerals carnallite, dolomite, and magnesite; in many rock-forming silicates; and as salts, such as magnesium chloride, in ocean and saline-lake waters. It is an essential constituent of animal and plant tissue.


Magnesium forms divalent compounds, chief among which are magnesium carbonate, which is formed by the reaction of a magnesium salt and sodium carbonate and is used as a refractory and insulating material; magnesium chloride, which is formed by reacting magnesium carbonate or oxide with hydrochloric acid and is used as dressing and filler for cotton and woolen fabrics, in paper manufacture, and in cements and ceramics; magnesium citrate, which is formed by the reaction of magnesium carbonate with citric acid and is used in medicine and effervescent beverages; magnesium hydroxide, formed by the reacting of magnesium salt and sodium hydroxide and used in medicine as the laxative “milk of magnesia,” and in sugar refining; magnesium sulfate, well known as Epsom salt; and magnesium oxide, called burnt magnesia, or magnesia, prepared by burning magnesium in oxygen or by heating magnesium carbonate and used as a heat-refractory and insulating material, in cosmetics, as a filler in paper manufacture, and as a mild, antacid laxative.

Alloyed forms of magnesium have considerable tensile strength. The metal is used when lightness is an essential factor: alloyed with aluminum or copper, it is used extensively in making castings for airplane parts; in artificial limbs, vacuum cleaners, and optical instruments; and in such products as skis, wheelbarrows, lawn mowers, and outdoor furniture. The unalloyed metal is used in photographic flash powders, incendiary bombs, and signal flares; as a deoxidizer in the casting of metals; and as a getter, a substance that achieves final evacuation in vacuum tubes.