: Calcium is an element with the symbol Ca , which has atomic number 20. Calcium is an alkaline metal substance. This is why calcium is a very reactive metal , which forms a dark oxide-nitride layer when it comes in contact with air. Most of its structural and chemical properties are similar to some of the heavier and more similar elements, strontium and barium. In terms of quantity, it ranks fifth among the elements of the Earth's crust. At the same time, it is the third most abundant metal in the world , after iron and aluminum. The most abundant calcium compound in the world is calcium carbonate , also known as limestone. It is found in the remains of fossils found in the oceans , which were formed in the early days of ocean formation. It is also a source of gypsum , anhydrite , fluorite and apatite calcium. The name Calcium comes from the Latin word Calx Lime . At that time it could be obtained by heating limestone.