Magnesium

Magnesium (pronounced /mæg?ni?zi?m/) is a chemical element with the symbol Mg, the atomic number 12, and an atomic mass of 24.31. Magnesium is the ninth most abundant element in the Earth by mass. It constitutes about 2% of the Earth’s crust by mass, and it is the third most abundant element dissolved in seawater. Magnesium ions are essential to all living cells, and is the 11th most abundant element by mass in the human body. The free element (metal) is not found in nature. Once produced from magnesium salts, this alkaline earth metal is used as an alloying agent to make aluminium-magnesium alloys, sometimes called “magnalium” or “magnelium”.

Notable characteristics

Elemental magnesium is a fairly strong, silvery-white, light-weight metal (two thirds the density of aluminium). It tarnishes slightly when exposed to air, although unlike the alkaline metals, storage in an oxygen free environment is unnecessary because magnesium is protected by a thin layer of oxide which is fairly impermeable and hard to remove. Like its lower periodic table group neighbor calcium, magnesium reacts with water at room temperature, though it reacts much more slowly than calcium. When it is submerged in water, hydrogen bubbles will almost unnoticeably begin to form on the surface of the metal, though if powdered it will react much more rapidly. Magnesium also reacts with hydrochloric acid (HCl) and produces heat and hydrogen when added to it. The magnesium will begin to bubble and become hot – too hot to touch comfortably. The reaction will occur faster with higher temperatures (see precautions). Magnesium is a highly flammable metal, but while it is easy to ignite when powdered or shaved into thin strips, it is difficult to ignite in mass or bulk. Once ignited it is difficult to extinguish, being able to burn in both nitrogen (forming magnesium nitride), and carbon dioxide (forming magnesium oxide and carbon).

Magnesium, when it burns in air, produces a brilliant white light. This was used in the early days of photography when magnesium powder was used as a source of illumination (flash powder). Later, magnesium ribbon was used in electrically ignited flash bulbs. Magnesium powder is still used in the manufacture of fireworks and marine flares where a brilliant white light is required.

Magnesium compounds are typically white crystals. Most are soluble in water, providing the sour-tasting magnesium ion Mg²⁺. Small amounts of dissolved magnesium ion contributes to the tartness and taste of natural waters. Magnesium ion in large amounts is an ionic laxative, and magnesium sulfate (Epsom salts) is sometimes used for this purpose. So-called “milk of magnesia” is a water suspension of one of the few insoluble magnesium compounds: magnesium hydroxide. The undissolved particles give rise to its appearance and name. Milk of magnesia is a mild base and is commonly used as an antacid.

Biology

Magnesium ions are essential to the basic nucleic acid chemistry of life, and thus are essential to all cells of all known living organisms. Plants have an additional use for magnesium in that chlorophylls are magnesium-centered porphyrins. Many enzymes require the presence of magnesium ions for their catalytic action, especially enzymes utilizing ATP, or those which use other nucleotides to synthesize DNA and RNA. Magnesium deficiency in plants causes late-season yellowing between leaf veins, especially in older leaves, and can be corrected by applying epsom salts (which is rapidly leached), or else crushed dolomitic limestone to the soil.

Magnesium is a vital component of a healthy human diet and deficiency has been implicated in a number of human diseases. Magnesium is readily available in many common foods but studies indicate that many Americans are magnesium deficient. Excess magnesium in the blood is freely filterd at the kidneys, and for this reason it is very difficult to overdose on magnesium from dietary sources alone . However, there are a number of magnesium supplements available which make overdose possible, particularly in people with poor renal function.

Isotopes

Magnesium has three stable isotopes: ²⁴Mg, ²⁵Mg, ²⁶Mg. All are present in significant amounts (see table of isotopes above). About 79% of Mg is ²⁴Mg. The isotope ²⁸Mg is radioactive and in the 1950s to 1970s was made commercially by several nuclear power plants for use in scientific experiments. This isotope has a relatively short half-life (21 hours) and so its use was limited by shipping times.

²⁶Mg has found application in isotopic geology, similar to that of aluminium. ²⁶Mg is a radiogenic daughter product of ²⁶Al, which has a half-life of 717,000 years. Large enrichments of stable ²⁶Mg have been observed in the Ca-Al-rich inclusions of some carbonaceous chondrite meteorites. The anomalous abundance of ²⁶Mg is attributed to the decay of its parent ²⁶Al in the inclusions. Therefore, the meteorite must have formed in the solar nebula before the ²⁶Al had decayed. Hence, these fragments are among the oldest objects in the solar system and have preserved information about its early history.

It is conventional to plot ²⁶Mg/²⁴Mg against an Al/Mg ratio. In an isochron dating plot, the Al/Mg ratio plotted is²⁷Al/²⁴Mg. The slope of the isochron has no age significance, but indicates the initial ²⁶Al/²⁷Al ratio in the sample at the time when the systems were separated from a common reservoir.