<P> Calcium is a chemical element with symbol Ca and atomic number 20 . An alkaline earth metal , calcium is a reactive pale yellow metal that forms a dark oxide - nitride layer when exposed to air . Its physical and chemical properties are most similar to its heavier homologues strontium and barium . It is the fifth most abundant element in Earth 's crust and the third most abundant metal , after iron and aluminium . The most common calcium compound on Earth is calcium carbonate , found in limestone and the fossilised remnants of early sea life ; gypsum , anhydrite , fluorite , and apatite are also sources of calcium . </P>
how abundant is calcium in the earth 's crust
the fifth most abundant element in Earth 's crust