What Is Strontium?

Strontium is a soft, silvery-white alkaline earth metal with atomic number 38 and chemical symbol Sr. It belongs to Group 2 of the periodic table, directly below calcium, and shares that group's characteristic reactivity: the metal burns in air and reacts vigorously with water. Strontium occurs naturally as four stable isotopes, with Sr-88 making up more than 82 percent of natural abundance. The element is relevant to engineering and applied physics through its isotopes, its role in specialty materials, and its radiological significance in nuclear environments.

Strontium was first identified in 1790 by Adair Crawford, who isolated a distinct mineral at Strontian, a Scottish village that gives the element its name. Humphry Davy obtained the pure metal in 1808 using electrolysis. Its natural occurrence is primarily in the minerals celestite (strontium sulfate) and strontianite (strontium carbonate), which serve as the principal commercial sources.

Physical and Chemical Properties

Strontium is softer than calcium and harder than barium within the alkaline earth series. Its melting point is 777 degrees Celsius and its density is 2.64 g/cm3. Freshly cut strontium displays a silvery metallic surface that rapidly yellows as oxide forms on exposure to air. Three allotropic forms exist, with transition points near 235 degrees and 540 degrees Celsius. The NIST atomic data for strontium documents ionization energies, energy levels, and spectroscopic data used in laser cooling and atomic clock research, where the narrow intercombination lines of strontium-87 and strontium-88 make the element a candidate for optical lattice clocks of exceptional precision.

Strontium-90 and Nuclear Applications

The radioisotope Sr-90, produced as a fission product in nuclear reactors and nuclear weapon detonations, has a half-life of approximately 29 years and emits high-energy beta radiation. As documented in the Los Alamos National Laboratory periodic table entry for strontium, Sr-90 is one of the most reliable long-lived beta emitters known and serves as the power source in SNAP (Systems for Nuclear Auxiliary Power) devices used on space vehicles, remote weather stations, and navigational buoys, where a compact source of sustained electrical power is required. The same isotope presents a radiological hazard because, as a calcium analog, it is incorporated into bone tissue following ingestion, where it can irradiate bone marrow over its multi-decade decay period. Environmental monitoring of Sr-90 concentrations in soil, water, and food is a standard component of nuclear site surveillance.

Advanced Materials and Industrial Uses

Strontium titanate, SrTiO3, is a synthetic compound with an exceptionally high refractive index and optical dispersion exceeding that of diamond, making it useful as an optical material and a model system for studying ferroelectric phase transitions in condensed matter physics. Strontium ferrite, SrFe12O19, is a low-cost, high-coercivity permanent magnet material produced in large quantities for motors, loudspeakers, and magnetic seals. The Royal Society of Chemistry periodic table entry for strontium notes that strontium compounds also produce the deep crimson color in pyrotechnics and signal flares, a use exploited since the nineteenth century, and that strontium chloride hexahydrate is the active ingredient in toothpastes formulated to reduce sensitivity in exposed dentinal tubules.

Applications

Strontium has applications in a wide range of disciplines, including:

• Nuclear power and space systems, where Sr-90 RTG units supply long-duration electrical power
• Optical and quantum metrology, where strontium atomic clocks contribute to precision timekeeping and navigation
• Permanent magnet manufacturing, using strontium ferrite for motors and sensors
• Medical imaging, where the positron-emitting isotope Sr-82 is used as a rubidium-82 generator for cardiac PET scans
• Pyrotechnics and safety signaling, where strontium salts produce high-visibility red flame