Calcium

Calcium, a reactive alkaline earth metal, has the symbol Ca and atomic number 20. Its properties resemble those of strontium and barium, its heavier homologues. Calcium ranks fifth in abundance in Earth's crust and third among metals, following iron and aluminium. Calcium carbonate, found in limestone and fossilized sea life remains, is the most common calcium compound on Earth. Other sources include gypsum, anhydrite, fluorite, and apatite. The name "calcium" originates from the Latin word calx, meaning "lime," obtained by heating limestone.