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sodium
Sodium is a chemical element with symbol Na (from New Latin ''natrium'') and atomic number 11. It is a soft, silver-white, highly reactive metal. In the Periodic table it is in column 1 (alkali metals), and shares with the other six elements in that column that it has a single electron in its outer shell, which it readily donates, creating a positively charged atom - a cation. Its only stable isotope is 23Na. The free metal does not occur in nature, but instead must be prepared from its compounds. Sodium is the sixth most abundant element in the Earth's crust, and exists in numerous minerals such as feldspars, sodalite and rock salt (NaCl). Many salts of sodium are highly water-soluble: sodium ions have been leached by the action of water from the earth's minerals over eons, so that sodium (and chlorine) are the most common dissolved elements by weight in the oceans.
Sodium was first isolated by Humphry Davy in 1807 by the electrolysis of sodium hydroxide. Many sodium compounds are useful, such as sodium hydroxide (lye) for soap-making and sodium chloride for use as a de-icing agent and a nutrient (edible salt).
Sodium is an essential element for all animals and some plants. Sodium ions are the major cation in the extracellular fluid (ECF) and as such are the major contributor to the ECF osmotic pressure, and thus ECF compartment volume. Loss of only water from the ECF compartment increases the sodium concentration: hypernatremia. Isotonic loss of water and sodium from the ECF compartment decreases the size of that compartment: ECF hypovolemia.
Sodium ions are pumped out of cells by Na+/K+-ATPase in exchange for potassium. It pumps three sodium ions out of the cell for every two potassium ions pumped in, contributing to the fact that about forty times as much potassium is inside cells compared to outside, and about ten times as much sodium is outside cells compared to inside. In nerve cells, a buildup of electrical charge across the cell membrane allows transmission of a nerve impulse - an action potential - when the charge is dissipated.
==Characteristics==
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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