Strontium
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.
Identity
- Atomic mass
- 87.621 u
- Category
- Alkaline earth metals
- Group
- 2
- Period
- 5
- Block
- s
- Oxidation States
- +2
- Phase (25 °C)
- Solid
Electronic Structure
- Electron configuration
- 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2
- Short configuration
- [Kr] 5s2
- Electrons per shell
- 2, 8, 18, 8, 2
- Electronegativity (Pauling)
- 0.950
- Electron affinity
- 5.02 kJ/mol
- 1st ionization energy
- 550 kJ/mol
- Ionization energies
- 550 · 1064 · 4138 · 5500 · 6910… kJ/mol
Physical properties
- Melting point
- 776.9 °C (1050 K)
- Boiling point
- 1376.8 °C (1650 K)
- Density
- 2.64 g/cm³
- Molar heat capacity
- 26.4 J/(mol·K)
Chemical behavior of strontium
Strontium has two valence electrons in the 5s subshell ([Kr] 5s2). It readily cedes both to form the stable Sr2+ cation with noble-gas configuration. A group 2 alkaline-earth metal between calcium and barium, it is more reactive than calcium but less than barium. It tarnishes quickly in air and burns with an intense scarlet-red flame.
Reactivity
Sr + 2 H₂O → Sr(OH)₂ + H₂
Reacts quickly and vigorously with liquid water releasing hydrogen and forming alkaline strontium hydroxide solution.
2 Sr + O₂ → 2 SrO
Reacts with ambient oxygen at room temperature giving strontium oxide and peroxide; heated, it burns with a bright red flame.
Sr + Cl₂ → SrCl₂
Combines directly with halogens such as chlorine in a strongly exothermic reaction to give strontium chloride.
Sr + 2 HCl → SrCl₂ + H₂
Dissolves very vigorously in dilute mineral acids with rapid hydrogen evolution.
Notable compounds
| Compound | Formula | Use or importance |
|---|---|---|
| Strontium carbonate | SrCO₃ | Pyrotechnics for crimson red, cathode-ray glass and ferrite magnets |
| Strontium nitrate | Sr(NO₃)₂ | Marine distress flares, red fireworks and tracer ammunition |
| Strontium chloride hexahydrate | SrCl₂·6H₂O | Desensitizing toothpastes by sealing dentine tubules; marine aquaria |
| Strontium ranelate | C₁₂H₆N₂O₈SSr₂ | Drug once used for severe osteoporosis by stimulating bone formation |
Electron shell distribution
View in the atom builderIsotopes
Natural strontium has four stable isotopes: strontium-88 (82.6 %), strontium-86 (9.9 %), strontium-87 (7.0 %, radiogenic from rubidium-87) and strontium-84 (0.6 %). The key radioisotope is strontium-90, a uranium fission product with a 28.8-year half-life emitting beta particles. Mimicking calcium, 90Sr deposits in bone if ingested, a grave radiotoxic hazard.
Occurrence and production
Fifteenth most abundant crustal element (about 360 ppm), richer than carbon and sulfur. Never free as metal. Main ores are celestine (SrSO4), forming bluish crystals, and strontianite (SrCO3). Commercial deposits lie in Spain, China and Mexico. Pure metal comes from aluminothermic reduction of the oxide or electrolysis of molten chloride.
History and origin of the name
In 1790 Adair Crawford recognized that strontianite from Strontian, Scotland differed from barium ores. Humphry Davy isolated strontium metal by electrolysis in 1808. The element was named after the village of Strontian. Its red flame soon made it a pyrotechnic staple.
Applications and relevance of strontium
Uses of the element
Metal is little used pure; its value is in compounds: red pyrotechnics, ferrite magnets, specialty glass, phosphors and toothpaste salts. Strontium-90 powers some radioisotope thermoelectric generators in remote devices.
Uses of its compounds
Carbonate and nitrate give crimson fireworks and flares; chloride desensitizes teeth; aluminate makes all-night glow materials; ranelate was an osteoporosis drug now restricted for cardiovascular risk.
Health, safety and environment
Biological role
Stable strontium is low toxicity; traces occur in bones by analogy with calcium. It is not an essential nutrient. Strontium-90 is dangerous because the body fixes it in bone matrix where beta radiation damages marrow, causing leukemia and bone tumors.
Safety
Metal ignites easily and must be stored under oil or inert gas away from water. Common salts have moderate toxicity but dust irritates. Any osteoporosis therapy with strontium must be decided by a physician.
Environment
Stable strontium is widespread and not a major pollutant, but mining waste must be managed. Strontium-90 fallout from atmospheric nuclear tests contaminated pastures and milk in the 20th century, which is why it was among the most feared contaminants.
Frequently asked questions about Strontium
Why do fireworks glow crimson red with strontium?
When powder burns, strontium chloride (SrCl) forms at high temperature. Electronic transitions of SrCl emit in a very narrow band around 635 nm, seen as dazzling crimson. Carbonate plus chlorine donors recreate this emitter in the flame.
Why was strontium-90 so feared in nuclear tests?
A fission product with 28.8-year half-life, it mimics calcium. Fallout fell on pastures, passed to cow milk and children, and fixed permanently in bone. There it emits continuous beta radiation damaging marrow, causing leukemias and osteosarcomas.
Why are strontium atomic clocks so precise?
They use laser-cooled atoms to measure an extremely stable light frequency, a tiny regular pendulum far steadier than everyday clocks. They aid navigation, communications and science tests. They could err by one second in billions of years.
Is strontium still used for osteoporosis?
Strontium ranelate was developed to cut fracture risk, acting on formation and loss of bone. Its use is restricted in many places over cardiovascular risk. Any osteoporosis treatment must be decided by a health professional.
How can archaeologists tell if someone grew up elsewhere?
Rocks and water of each region carry characteristic strontium isotope ratios. Growing children fix traces into tooth enamel from food and water. Comparing enamel of childhood with the burial place tells whether the person grew locally or migrated.
How does strontium relieve sensitive teeth?
Sensitivity appears when worn enamel exposes microscopic tubules to the nerve. Brushing with strontium salts reacts with saliva forming crystals that plug the holes. Sealed tubules no longer transmit cold or heat to the nerve.
Why do some glow objects shine all night?
Classic glow materials fade in minutes. Modern strontium aluminate phosphors trap daylight energy in their structure and release green-blue light slowly for over twelve hours.
Main sources consulted: PubChem, Royal Society of Chemistry, NIST, IUPAC. Last revision: November 2026.