Strontium – facts and properties
- Symbol
- Sr
- Atomic number
- 38
- Atomic mass
- 87.62 u
- Period
- 5
- Group
- 2
- Category
- alkaline earth metal
- Electron configuration
- [Kr] 5s²
- Electronegativity
- 0.95
- Atomic radius
- 219 pm
- Valence electrons
- 2
- State at standard conditions
- solid
- Melting point
- 777 °C
- Boiling point
- 1382 °C
- Density
- 2.64 g/cm³
- Discovered
- 1790
Strontium is a soft, silvery-yellowish alkaline earth metal with atomic number 38. It is named after the Scottish village of Strontian. Most people know strontium only from fireworks, because its compounds colour flames a bright red. The element resembles calcium, so the body builds it into bones, which is a problem in the case of radioactive strontium-90.
Position in the periodic table
Occurrence and extraction
Strontium is not rare in the Earth's crust, being considerably more abundant than copper, for example, but it never occurs pure. The most important minerals are celestine, a strontium sulfate, and strontianite, a strontium carbonate. Celestine mainly forms deposits in sedimentary rocks, while strontianite often occurs in mineral veins.
Strontianite was found in the eighteenth century in lead mines near Strontian in Scotland. At first it was taken for a variety of a mineral already known. Around 1790 chemists, among them Adair Crawford, recognised that it contained an element of its own. The metal itself was later produced by the English chemist Humphry Davy by electrolysis.
Today the carbonate or oxide is first obtained from the minerals. The metal is made from it by reduction with aluminium or by electrolysis of a melt, and only small quantities are needed.
Properties and reactions
Strontium is in period 5 and group 2, an alkaline earth metal between calcium and barium. Its electron configuration is [Kr] 5s², with 2 valence electrons. The electronegativity is 0.95, so it gives up its two outer electrons easily. The metal has a density of 2.64 g/cm³, melts at 777 °C and boils at 1382 °C. It is soft enough to cut, but quickly turns yellowish in air as oxide and nitride form.
With water, strontium forms strontium hydroxide and hydrogen, reacting more strongly than calcium and less violently than barium. It is therefore stored under oil. In compounds it carries a double positive charge. Most of its salts are colourless. Strontium carbonate and strontium sulfate are sparingly soluble, while strontium nitrate and strontium chloride dissolve well.
In a flame, strontium gives a crimson colour. The reason lies in electron structure: heat lifts electrons to higher energy levels, they give off light of particular colours as they drop back. For strontium the strong red comes mainly from short-lived molecules of strontium with chlorine or with oxygen and hydrogen that form in the hot flame. The same principle is used in the flame test in chemistry lessons to tell strontium apart from calcium and barium.
Uses in everyday life and technology
Fireworks are the best-known use. Strontium salts such as the nitrate and the carbonate give the bright red of flares, star shells and signal torches.
Other uses are technical. Strontium ferrite is a hard magnetic material for cheap permanent magnets, for instance in loudspeakers, fridge doors and small electric motors. Strontium aluminate doped with rare earths glows in the dark and is the basis of modern glow pigments, found on watch dials and emergency exit signs. The glass of old television tubes contained strontium oxide to block X-rays. High-precision optical clocks use strontium atoms as their frequency reference and are among the most accurate timekeepers in the world.
Medically, strontium drew attention because it resembles calcium. Strontium ranelate was used against osteoporosis. Because of risks to the heart and circulation its use was strongly restricted in the EU, and the drug is no longer on the market. Some toothpastes for sensitive teeth contain strontium salts that are meant to seal tiny channels in dentine. How well this works is disputed among scientists.
The isotope strontium-90 is a special case. It forms in nuclear fission, for example in atomic bomb tests or reactor accidents such as Chernobyl, and emits beta radiation. Its half-life is roughly thirty years. Because the body treats strontium like calcium, strontium-90 settles in bone and can irradiate it for a long time. Stable strontium, by contrast, is only slightly toxic. At school, strontium appears only in the flame test, with a tiny amount of a salt; strontium metal does not belong in a classroom.
Molecules containing this element
- Strontium chloride SrCl2
- Strontium peroxide SrO2
- Strontium carbonate SrCO3
Quick question
What is the atomic number of strontium (Sr)?
FAQ
Why does strontium make fireworks red?
When heated, electrons of the strontium atoms are lifted to higher energy levels. When they fall back they emit light. For strontium the red comes mainly from short-lived strontium compounds formed in the hot flame, so strontium salts glow red. Pyrotechnicians mostly use strontium nitrate or strontium carbonate.
Is strontium radioactive?
Natural strontium is not radioactive; it consists of stable isotopes. The radioactive one is strontium-90, an artificial fission product from nuclear reactors and atomic bomb tests. It settles in bone like calcium, which is why it is watched closely after accidents.
What is strontium used for?
Strontium compounds colour fireworks red, form permanent magnets as strontium ferrite and glow-in-the-dark pigments as strontium aluminate. It used to be in the glass of tube televisions. As a drug and in toothpaste it plays a minor role.