Calcium

The chemical element calcium is a malleable, light, silver while metal, an Alkaline Earth Metal of Group IIA in the periodic table. Its symbol is Ca, its atomic number is 20, and its atomic weight is 40.08.

Calcium was first prepared by Sir Humphrey Davy in 1808. following the method of J.J. Berzellus and M.M. Pontin. Davy electrolyzed a mixture of lime, CaO, and mercury to produce an amalgam (a mercury, he obtained a calcite, a form of calcium carbonate (CaCO3).

Occurrence
In cosmic abundance calcium is 13th among the elements; on Earth it ranks 5th and forms 3.2 % of the Earth’s crust, being less prevalent than aluminum (7.3%) or Iron (4.1%). It is not found free in nature but is common as the carbonate rock limestone, CaCO3. It is also well distributed as the minerals calcium phosphate, silicate, fluoride, and sulfate. As calcium-magnesium carbonate it is one of the principal components of dolomite minerals and is found in pearls, coral, natural chalk, calcite, onyx and marble.

Chemical Properties
Calcium almost always has an Oxidation number of +2. it reacts readily as a reducing agent with most nonmetals, reacting with all halogens (X2) as follows:

Ca + X2 -----> CaX2

When heated with nitrogen it forms calcium nitride, Ca3N2, and when heated with oxygen it burns with a brilliant light to give the oxide, CaO. Calcium reacts spontaneously with water and acids to liberate hydrogen gas.

Biological Functions
Resides being a major mineral in such hard biological structures as shells, bones, and teeth, calcium plays other important roles in the biochemistry of most organisms. In the human body, which is about 2% calcium by weight, about 99% of this calcium occurs in the bones and teeth and the remainder into muscle contraction and hence to cardiac function. Calcium ions are also essential in the transmission of nerve impulses and in blood coagulation, and their roles in processes such as vision are the subject of ongoing research.

Parathyroid and thyroid hormones help to maintain proper calcium balance in tissues. A lack of calcium can impair growth and lead to such conditions as rickets and tetany. Milk, milk product, leafy green vegetables, and shellfish are sources of dietary calcium.

Production
An important method of producing pure calcium is by the reduction of calcium chloride using metallic aluminum, according to the reaction:

3 CaCl2 + 2 Al ----> 3 Ca + 2 AlCl3

It is also produced by the electrolysis of fused calcium chloride at 800oC. As it forms, the light molten calcium metal floats to the surface, where it is continuously withdrawn.

Important Compounds
Because limestone – calcium chloride, CaCO3 is so abundant in nature and to readily converted to other compounds, it is the most important calcium compound. The reaction of calcium with hydrochloride and

CaCO3 + 2 HCl ---->; CaCl2 + CO2 + H2O

Is typical of reactions with acids that produce calcium compounds containing sucah anions as phosphate, acetate, nitrate or oxalate.

Decomposition of limestone by heating is an inexpensive method for the production of lime, CaO.

CaCO3 -----> CaO + CO2

Lime has been used since ancient times in making mortar. When lime react with water in the process called slaking, 15.96 kcal of heat is liberated for each mole, and calcium hydroxide, Ca(OH)2, is formed. Ordinary mortar and some plasters are mixture of calcium hydroxide, sand, and water. Exposure to air causes evaporation of the water, the mortar hardens, and in the course of time reaction with the carbon dioxide of the atmosphere reforms calcium carbonate, the starting material.

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