Strontium is a chemical element with symbol Sr and atomic number 38. An alkaline earth metal, strontium is a soft silver-white or yellowish metallic element that is highly reactive chemically. The metal turns yellow when it is exposed to air. Strontium has physical and chemical properties similar to those of its two vertical neighbors calcium and barium. It occurs naturally in the minerals celestine, putnisite and strontianite. While natural strontium is stable, the synthetic ⁹⁰Sr isotope is present in radioactive fallout and has a half-life of 28.90 years.