← Periodic table · Alkaline earth metals
Strontium (Sr)
Strontium (symbol Sr) is the chemical element with atomic number 38: an alkaline earth metal, solid at 0 °C, with an atomic mass of 87.62 u. Discovered in 1790 (Adair Crawford).
Strontium is an alkaline earth metal, a cousin of calcium. Its salts turn fireworks and distress flares a vivid red. Its radioactive isotope, strontium-90, is a dreaded fallout pollutant from nuclear tests that lodges in bones.
Key facts
- Atomic number
- 38
- Symbol
- Sr
- French name
- Strontium
- Atomic mass
- 87.62 u
- Molar mass
- 87.62 g/mol
- Family
- Alkaline earth metals
- Position
- period 5, group 2, s-block
- State at 0 °C
- Solid
- Melting point
- 776.9 °C
- Boiling point
- 1,376.9 °C
- Density
- 2.64 g/cm³
- Electronegativity (Pauling)
- 0.95
- First ionization energy
- 550 kJ/mol
- Atomic radius
- 200 pm
- Electron configuration
- [Kr] 5s²
- Abundance in Earth's crust
- 370 ppm (15th most abundant element)
- Indicative price
- 7 $/kg (order of magnitude)
- Radioactive
- no
- Discovery
- 1790, Adair Crawford
Origin of the name
From Strontian, a village in Scotland.
What does strontium look like?
A soft, silvery metal that quickly yellows in air as it coats itself with oxide. It reacts with water and must be stored under oil.
Where is strontium found?
About 370 ppm of Earth's crust, making it a fairly common element. Seawater contains nearly 8 mg per liter. It is concentrated in two main minerals, celestine and strontianite.
Natural sources
- Celestine
- Strontianite
How is strontium produced?
Celestine (strontium sulfate) is quarried and then converted into carbonate. The pure metal, rarely needed, is obtained by reduction with aluminium. World celestine output is on the order of 300,000 to 400,000 tonnes a year.
Top producers
- Spain
- Iran
- China
- Mexico
What is strontium used for?
- The red of fireworks and distress flares.
- Strontium ferrite magnets (speakers, small motors).
- Glow-in-the-dark strontium aluminate pigments.
- Formerly: glass for CRT televisions, to block X-rays.
- Strontium optical clocks, among the most precise in the world.
In everyday life
- Red firework
- Fridge or speaker magnet
- Glow-in-the-dark watch hands
- Toothpaste for sensitive teeth (some formulas)
Role in living things
With no essential role, it is treated by the body like calcium and settles in bones; an adult contains about 300 mg. Strontium ranelate was prescribed for osteoporosis before being restricted over its cardiovascular side effects.
Hazards of strontium
Stable strontium has low toxicity. Strontium-90, produced by nuclear fission, has a half-life of 29 years and builds up in bones, where it irradiates the bone marrow.
Cosmic origin
- Dying stars: Made slowly in red giants (stars like the Sun at the end of their lives).
- Supernovae: Forged in the cores of massive stars, then scattered when they explode.
History of strontium
In 1790, Adair Crawford noticed that a mineral from the lead mines of Strontian, Scotland, differed from the others. It was then studied by Thomas Charles Hope, and Humphry Davy isolated the metal by electrolysis in 1808.
Did you know?
- In the late 1950s, a survey in St. Louis, Missouri, collected hundreds of thousands of baby teeth to measure strontium-90 from nuclear tests; it helped bring about the 1963 ban on atmospheric testing.
- Strontian is one of the few places in the United Kingdom to have given its name to an element.
- Modern glow-in-the-dark toys and signs shine for hours thanks to strontium aluminate.
Isotopes of strontium
Four stable isotopes. Strontium-90, a product of nuclear tests and accidents, is feared because it lodges in bone just like calcium.
- strontium-82: radioactive, half-life 25.4 days. Parent in the rubidium-82 generator used for heart imaging.
- strontium-84: 0.56% of natural strontium, stable
- strontium-86: 9.86% of natural strontium, stable
- strontium-87: 7% of natural strontium, stable. Partly born from rubidium-87, it dates rocks and traces where food comes from.
- strontium-88: 82.58% of natural strontium, stable
- strontium-89: radioactive, half-life 50.6 days. Relieves the pain of bone metastases.
- strontium-90: radioactive, half-life 28.9 years. A major radioactive fallout product; it once powered remote electric generators.
Related elements
Same family (alkaline earth metals): Beryllium (Be), Magnesium (Mg), Calcium (Ca), Barium (Ba), Radium (Ra).
Same column (group 2): Beryllium (Be), Magnesium (Mg), Calcium (Ca), Barium (Ba), Radium (Ra).