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Calcium (Ca). Diagram of the nuclear composition and electron configuration of an atom of calcium-40 (atomic number: 20), the most common isotope of this element. The nucleus consists of 20 protons (red) and 20 neutrons (blue). The stability of an element's outer electrons determines its chemical and physical properties. Calcium, vital for cellular function, is an alkaline earth metal in group 2, period 4, and the s-block of the periodic table. In elemental form, it is a reactive metal that burns with a bright orange-red flame when heated in air.
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