Explanation:
Magnesium (Mg, Z=12).
Magnesium is a silver-white, relatively soft metal. The name of the element is derived from Magnesia, a district in Thessaly in central Greece. It is found in the Earth's crust at a concentration of 2.3%, making it the 7th most abundant element. Large amounts of magnesium are also present in the minerals in the Earth's mantle. It is obtained from seawater, carnalite [MgKCl3·6H2O], dolomite [a mixture of calcium and magnesium carbonate, CaMg(CO3)2], and magnesite [magnesium carbonate, MgCO3].
Magnesium alloyed with aluminum and traces of other metals is used in car and aircraft construction; magnesium alloys are also used in other lightweight devices, such ladders, cameras, bicycle frames, hard disk drives, etc. Magnesium is more easily oxidized than iron, and is used in sacrificial anodes to protect iron pipes and other structures that corrode easily.
Magnesium burns in air with a brilliant white flame, and is used in fireworks and incendiary bombs. (It used to be used in disposable flashbulbs, but this use has been supplanted by other types of illumination.) Magnesium fires are very difficult to put out, since even in the absence of air, burning magnesium reacts with nitrogen to form magnesium nitride (Mg3N2), and with water to produce magnesium hydroxide and hydrogen gas.
Magnesium is found in a number of familiar compounds. Magnesium oxide, MgO, is used in refractory bricks that are capable of withstanding the high temperatures in fireplaces and furnaces (magnesium oxide melts at 2800 °C). Magnesium sulfate heptahydrate, MgSO4·7H2O, better known as Epsom salt, is a muscle relaxant and a mild laxative. Magnesium hydroxide, Mg(OH)2, also known as milk of magnesia, is a laxative and antacid. (The "milk" in "milk of magnesia" refers to the fact that since magnesium hydroxide is not very soluble in water, it tends to form a chalky, white suspension that looks like milk — but with considerably different physiological effects.)
Green plants contain a molecule called chlorophyll, which consists of a flat ring of carbon and nitrogen atoms with a large open space in the middle, in which a magnesium ion is bound, held in place by the nitrogen atoms. The chlorophyll molecule absorbs light from the sun, and in the process of photosynthesis, the energy from the light is converted into chemical energy that the plant can use to power a multitude of processes.
In organic chemistry, magnesium reacts with bromoalkanes (hydrocarbons containing carbon-bromine bonds) to form organomagnesium compounds known as Grignard reagents (after their discoverer, Victor Grignard, who won a Nobel Prize in Chemistry, 1912). These compounds are extremely useful in forming new carbon-carbon bonds, and are often used in the synthesis of organic compounds. Grignard reagents are notoriously sensitive to water, and care must be taken to ensure that the apparatus in which the reaction is being carried out is extremely dry.
