A chemical element with atomic number 38. Strontium is a silver-gray metal that is highly reactive with water and air. It occurs on Earth in compounds, mainly in the minerals celestine and strontianite. Strontium isotope ⁹⁰Sr is produced in fission reactions and is radioactive with a half-life of 29 years. It is part of spent fuel but can also be used as a heat source in radioisotope thermoelectric generators. Because strontium is chemically similar to calcium, it can be deposited in bones and teeth.