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. 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.
Strontium is a grey, silvery metal that is softer than calcium and even more reactive toward water, with which it reacts on contact to produce strontium hydroxide and hydrogen gas. It burns in air to produce both strontium oxide and strontium nitride, but since it does not react with nitrogen below 380 °C, at room temperature it will only form the oxide spontaneously.
The human body absorbs strontium as if it were calcium. Strontium has been shown to inhibit sensory irritation when applied topically to the skin.