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Calcium (eingedeutscht Kalzium geschrieben) ist ein chemisches Element mit dem Elementsymbol Ca und der Ordnungszahl 20. Im Periodensystem steht es in der zweiten Hauptgruppe bzw. der 2. IUPAC-Gruppe und zählt daher zu den Erdalkalimetallen. Die Schreibweise Calcium entspricht der IUPAC-Norm und gilt als fachsprachlich.
Elementares Calcium ist ein glänzend silberweißes, weiches und duktiles Metall. In der Erdhülle kommt Calcium nur in gebundener Form vor und ist, auf den Massenanteil (ppmw) bezogen, nach Sauerstoff, Silicium, Aluminium und Eisen das fünfthäufigste Element. Auch ist es das häufigste Metall im menschlichen Körper.