钙(calcium)
元素周期表IIa主族硷土金属元素，化学符号Ca，原子序数20。为人体内含量最多的金属元素，存在於骨骼和牙齿之中，具有许多功能（参阅calcium deficiency）。在地壳中的丰度占第五位，但在自然界不能以游离状态存在。钙以方解石（碳酸钙）的形式存在於石灰岩、白垩、大理石、白云石、蛋壳、珍珠、珊瑚、钟乳石、石笋和许多海洋动物的壳体中。在自然界以磷灰石矿形式存在，还存在於许多其他矿物中。可作为合金添加剂和应用在其他冶金方面。与铅组成的合金可用作电话电缆包皮和蓄电池铅板。碳酸钙可作为石灰的来源、填料、中和剂和混合剂。合成的碳酸钙常用於纯度要求很高的医药上（抗酸剂、饮食补充钙、牙膏）和食物中（作发酵粉）以及实验室化学合成等。加水後的氧化钙（又称石灰或生石灰）和其产品，以及氢氧化钙在工业应用上很重要。其他重要的化合物包括氯化钙（乾燥剂）、次氯酸钙（漂白剂）、硫酸钙（石膏、熟石膏），以及磷酸钙（主要用作植物养料和塑胶稳定剂）。
English version：
calcium
Chemical element, one of the alkaline earth metals, chemical symbol Ca, atomic number 20. The most abundant metallic element in the human body, it is stored in bones and teeth and has many functions (see calcium deficiency). It is the fifth most abundant element in the earth's crust but does not occur naturally in the free state. Its compounds, in which it has valence 2, include limestone, chalk, marble, and dolomite. It occurs in eggshells, pearls, coral, and many marine shells as calcium carbonate, or calcite, in apatite (as calcium phosphate), in gypsum (as calcium sulfate), and in many other minerals. It is used as an alloying agent and in other metallurgical applications; its alloy with lead is used as cable sheathing and grids for batteries. Calcite is used as a lime source, filler, neutralizer, and extender; in pure form it is used as an antacid and calcium supplement and in baking powder. Calcium oxide (lime) and its product after water addition, calcium hydroxide (slaked lime), are important industrially. Other important compounds are the chloride (a drying agent), the hypochlorite (a bleach), the sulfate (gypsum and plaster of paris), and the phosphate (a plant food and stabilizer for plastics).