Calcium is the chemical element of atomic number 20. Its symbol is Ca. It is a light earth alkaline metal, gray, used as a reducing agent for mining torsion, uranium and zirconium; When exposed to air, forms a layer of dark oxide. Its physical and chemical properties are similar to its heavier homologues, strontium and barium. It is the fifth most abundant element of the earth's crust and the third most abundant metal, after iron and aluminum. The most common calcium compound found on Earth is calcium carbonate, found in limestone and in fossils dating back to the anesthetic marine life; Plaster, anhydrite, fluorite and apatite are also sources of calcium.
The name derives from the Latin calx, "lime", meaning obtained from the heating of the limestone. Its compounds have been known since antiquity even though their chemistry was unknown until the 17th century. It was first isolated by Humphrey Davy in 1808 by electrolysis of its oxide. While pure metal can not boast many applications due to its high reactivity, it is often used in small quantities as a component of steel alloys, while some lead and calcium alloys are sometimes used in the manufacture of automotive batteries. Calcium compounds are, moreover, widespread in many areas: for example, they are used in the food industry, in the pharmaceutical industry, in the paperboard such as bleach, cement, soap production, and as electrical insulators.
Calcium is the fifth most abundant element of the human body and the most abundant metal. Calcium ions play a vital role in physiology and biochemistry of the body and cell as electrolytes. They play an important role in the signal transduction pathways, where they act as a second messenger, in the release of neurotransmitters from neurons, in contraction of all types of muscle cells and in fertilization. Many enzymes require calcium ions as cofactors. External calcium ions also are important in maintaining the potential difference between excitable cell membranes and proper bone formation.
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