Calcium: the essentials
Calcium as the element is a grey silvery metal. The metal is rather hard. Calcium is an essential constituent of leaves, bones, teeth, and shells. Calcium is the fifth most abundant element in the earth's crust and makes up more than 3% of the crust. Calcium does not occur as the metal itself in nature and instead is found in various minerals including as limestone, gypsum and fluorite. Stalagmites and stalactites contain calcium carbonate (CaCO3). Calcium carbonate is the basis of the cement industry.
Calcium is classified chemically as one of the alkaline earth elements (that is, in Group 2 of the periodic table. The metal is rather reactive. It readily forms a white coating of calcium nitride (Ca3N2) in air. It reacts with water and the metal burns with a yellow-red flame, forming largely the nitride.
Calcium: historical information
Compounds such as lime (CaO, calcium oxide) were prepared by the Romans in the first century under the name calx. Literature dating back to about 975 AD notes that plaster of paris (calcium sulphate, CaSO4, dehydrated gypsum) is useful for setting broken bones. Other calcium compounds used in early times include limestone (CaCO3, calcium carbonate).
Calcium metal was not isolated until 1808. After learning that Berzelius and Pontin prepared calcium amalgam by electrolysing lime in mercury, Sir Humphry Davy was able to isolate the impure metal. He did this by the electrolysis of a mixture of lime and mercuric oxide (HgO). Calcium metal was not available in large scale until the beginning of the 20th century.
Sometime prior to the autumn of 1803, the Englishman John Dalton was able to explain the results of some of his studies by assuming that matter is composed of atoms and that all samples of any given compound consist of the same combination of these atoms. Dalton also noted that in series of compounds, the ratios of the masses of the second element that combine with a given weight of the first element can be reduced to small whole numbers (the law of multiple proportions). This was further evidence for atoms. Dalton's theory of atoms was published by Thomas Thomson in the 3rd edition of his System of Chemistry in 1807 and in a paper about strontium oxalates published in the Philosophical Transactions. Dalton published these ideas himself in the following year in the New System of Chemical Philosophy. The symbol used by Dalton for calcium is shown below. [See History of Chemistry, Sir Edward Thorpe, volume 1, Watts & Co, London, 1914.]
Calcium: physical properties
Calcium: orbital properties
Isolation: calcium metal is readily available commercially and there is no need to make it in the laboratory. Commercially it can be made by the electrolysis of molten calcium chloride, CaCl2.
cathode: Ca2+(l) + 2e- → Ca
anode: Cl-(l) → 1/2Cl2 (g) + e-
The calcium chloride is made by the action of hydrochloric acid upon calcium carbonate. Calcium chloride is also a byproduct in the Solway process used to make sodium carbonate.
CaCO3 + 2HCl → CaCl2 + H2O + CO2
Alternatively, and on small scale, calcium can be made through the reduction of CaO with aluminium or of CaCl2 with sodium metal
6CaO + 2Al→ 3Ca + Ca3Al2O6
CaCl2 + 2Na→ Ca + 2NaCl
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