Calcium is a chemical element with symbol Ca and atomic number 20. Calcium is a soft gray alkaline earth metal, fifth-most-abundant element by mass in the Earth's crust. The ion Ca2+ is also the fifth-most-abundant dissolved ion in seawater by both molarity and mass, after sodium, chloride, magnesium, and sulfate. Calcium is one of the seven basic building blocks of most of the Earth's crust, and constitutes about 99.5% of the Earth's crust. Calcium is a lustrous, silver-white, soft, alkaline earth metal. It is found in numerous minerals, such as limestone, sand, and limestone. Calcium is never found in nature as a free element, although it is found as a highly reactive metal in seawater (not in air). Calcium is needed for various calcium-based compounds, and is also used in the corrosion of rust.