- Calcium (Ca) is a silvery-white, moderately reactive alkaline earth metal with atomic number 20, positioned in Group 2 of the periodic table.
- It has two valence electrons in its outermost shell, which it readily loses to form Ca²⁺ cations. Its atomic structure consists of twenty protons, typically twenty neutrons, and twenty electrons arranged in four shells (two in the first shell, eight in the second, eight in the third, and two in the fourth).
- Calcium has six stable isotopes: calcium-40 (⁴⁰Ca), the most abundant at about 96.941%; calcium-44 (⁴⁴Ca), about 2.086%; calcium-42 (⁴²Ca), about 0.647%; calcium-48 (⁴⁸Ca), about 0.187%; calcium-43 (⁴³Ca), about 0.135%; and calcium-46 (⁴⁶Ca), about 0.004%. The rare isotope ⁴⁸Ca is notable for being doubly magic (having full proton and neutron shells) and for its use in nuclear physics experiments.
- Calcium is the fifth most abundant element in Earth’s crust and the most abundant alkaline earth metal. It occurs naturally only in ionic compounds, primarily as calcium carbonate (CaCO₃) in limestone, chalk, and marble; calcium sulfate (CaSO₄·2H₂O) in gypsum; and calcium fluoride (CaF₂) in fluorite. Large deposits of these minerals are found worldwide and form the basis for calcium’s industrial production, usually by reduction of lime (CaO) with aluminum or by electrolysis of molten calcium chloride.
- The element was first isolated in 1808 by Sir Humphry Davy, who obtained it by electrolyzing a mixture of lime (CaO) and mercuric oxide. The name “calcium” derives from the Latin calx, meaning “lime” or “limestone,” reflecting its common occurrence in these forms.
- Biologically, calcium is essential for life and is the most abundant metal in the human body. About 99% of the body’s calcium is stored in bones and teeth as hydroxyapatite [Ca₁₀(PO₄)₆(OH)₂], providing structural strength. The remaining calcium ions play critical roles in muscle contraction, nerve signal transmission, blood clotting, and cellular signaling pathways. Calcium homeostasis is tightly regulated by hormones such as parathyroid hormone (PTH), calcitonin, and vitamin D. In plants, calcium is important for cell wall stability and membrane function.
- Industrially, calcium compounds have wide-ranging applications. Calcium carbonate is used in construction materials, cement, and as a filler in paper and plastics. Calcium oxide (quicklime) and calcium hydroxide (slaked lime) are used in steelmaking, water treatment, and pH control. Calcium chloride serves as a de-icing agent and drying agent, while calcium sulfate is a key ingredient in plaster and drywall. Metallic calcium is used as a reducing agent in the extraction of other metals and in the production of certain alloys.
- Chemically, calcium reacts readily with water, though less violently than alkali metals, producing calcium hydroxide and hydrogen gas. It also reacts with oxygen to form calcium oxide and with halogens to produce calcium halides. Its compounds are generally white solids with high melting points and low solubility, though calcium chloride is highly soluble.
- Environmentally and nutritionally, calcium is vital but must be consumed in appropriate amounts. Deficiency can lead to osteoporosis, rickets, and impaired muscle function, while excess intake may cause kidney stones and interfere with the absorption of other minerals. In aquatic systems, calcium is important for buffering pH and supporting shell formation in marine life.
