Atomic number 38

Strontium

Sr
+ phase
− phase

Shells Orbitals 5s

Drag to rotate; pinch or Ctrl + scroll to zoom. Arrow keys rotate, + − zoom, R resets.

38 electrons in 5 shells: 2, 8, 18, 8, 2.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Alkaline earth metal
Period: 5
Group: 2
Block: s
Standard atomic weight: 87.62(1) Da
Electron configuration: [Kr] 5s²
Phase at 25 °C: solid
Melting point: 777 °C
Boiling point: 1377 °C
Density: 2.64 g/cm³
Electronegativity (Pauling): 0.95
Discovered: 1790, Adair Crawford

Neighbours in the table

Rb Y

Strontian, a town in Scotland, gave strontium its name, and the mineral strontianite came from its mines. In 1790 the Irish chemist Adair Crawford was working with witherite, a barium mineral, and got unexpected results when he added hydrochloric acid. Jefferson Lab’s account in PubChem’s record says he blamed an unknown mineral in the sample and called it strontianite. The CIAAW history in the same record credits Thomas Charles Hope instead, in 1792, who noticed the element’s vivid red flame. Humphry Davy isolated the metal in 1808 by electrolysing strontium chloride mixed with mercuric oxide.

Strontium needs five shells for its 38 electrons, 2, 8, 18, 8 and 2, written [Kr] 5s². Like calcium and barium, it readily gives up both outer electrons in its compounds, leaving Sr²⁺. Its 5s pair lies a shell farther from the nucleus than calcium’s, and OpenStax explains that an electron farther out is easier to remove. So the first ionisation energy falls to 5.695 eV, and reactivity rises with it.

Los Alamos describes a cut surface that starts silvery and quickly yellows as oxide forms, so the metal is kept under kerosene. The same page lists three allotropes, forms of the solid that change over at 235 °C and 540 °C.

Where it is found

Jefferson Lab puts strontium at 370 milligrams per kilogram of the crust and 7.9 milligrams in each litre of seawater. Los Alamos names its two chief minerals: celestite, the sulfate SrSO₄, and strontianite, the carbonate SrCO₃. OpenStax lists strontium sulfate among the sulfates that dissolve only slightly in water. Many group 2 salts behave this way because the doubly charged metal ion gives them high lattice energies.

Celestite is what gets mined. The US Geological Survey (USGS) counts about 510,000 tons worldwide in 2024, with Spain and Iran at 200,000 tons each, then China at 80,000 and Mexico at 25,000. The United States has deposits in many places but has mined none since 1959, and its large-scale domestic production of strontium carbonate stopped in 2006.

OpenStax gives electrolysis as the common way to make the reactive group 2 metals, and Los Alamos describes it for strontium: the molten chloride, mixed with potassium chloride. It also names a second route, heating strontium oxide with aluminium in a vacuum until the strontium distils off.

Why it matters

In the United States, 2024 use, estimated from imports, split into ceramic ferrite magnets, 40 per cent; pyrotechnics and signals, another 40; drilling fluids, 2; and other uses, 18. For the magnets, strontium carbonate is sintered, heated until the grains fuse, with iron oxide. Barium can stand in, the USGS notes, but the barium magnet cannot run as hot.

Rocks keep a second record in strontium. Rubidium-87 decays to strontium-87 with a half-life of 4.97 × 10¹⁰ years, in the International Atomic Energy Agency’s evaluation. The ratio of strontium-87 to strontium-86 varies among rocks, soils and waters and carries over into plants and animals. Analysts use it to trace where food was grown, in archaeology, in crime-scene work and in human migration.

IUPAC notes that strontium’s stable isotopes fractionate, or sort by mass, only slightly. It gives two causes: their masses differ by small fractions, and strontium takes part in no redox reactions in ordinary surface environments. Seawater’s ratio shifts with seafloor spreading, ridge vents and continental weathering, so marine deposits can be dated against a global chart.

Strontium-82, half-life 25.35 days, serves heart imaging by PET, positron emission tomography. The USGS records that in 2024 a strontium atomic clock was judged the most accurate timepiece to date.

Properties

Properties of strontium Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	38
Standard atomic weight	87.62(1) Da	Da	IUPAC/CIAAW 2024
Melting point	777 °C	°C	Los Alamos: 777 °C
Boiling point	1377 °C	°C	Los Alamos: 1377 °C
Density	2.64 g/cm³	g/cm³
Electronegativity (Pauling)	0.95
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s²
Phase at 25 °C	solid

Safety

Strontium metal carries the signal word Danger in PubChem’s GHS record. Of 304 company reports to ECHA, the European Chemicals Agency, 80.6% say that with water it gives off flammable gas able to ignite by itself. About half add severe skin burns, eye damage and possible harm to an unborn child, while Haz-Map calls natural strontium not radioactive and not harmful on skin or when breathed. For anyone handling the metal, the PubChem Laboratory Chemical Safety Summary is the sheet to consult.

Questions

Why do fireworks turn red with strontium?

The red comes from strontium compounds: the US Geological Survey names strontium nitrate as the one that colours fireworks and signal flares, and Jefferson Lab, in PubChem's record, adds the carbonate. OpenStax explains the colour as part of the emission spectrum, the particular wavelengths hot atoms give out, and groups strontium with calcium as giving shades of red, while barium gives green. Pyrotechnics and signals took about 40 per cent of the strontium used in the United States in 2024.

Is strontium radioactive?

Natural strontium is not: Los Alamos describes it as a mixture of four stable isotopes. Strontium-90 forms in nuclear reactors and in nuclear weapon explosions, according to ATSDR, the US Agency for Toxic Substances and Disease Registry. Its half-life is 29 years for Los Alamos and ATSDR, about 28.8 for Jefferson Lab and 28.91 in the Atomic Mass Data Center table, both in PubChem's record. It decays by beta emission to yttrium-90 and, once absorbed, collects in bone.

What happens when strontium is put in water?

It reacts, giving off hydrogen gas and forming strontium hydroxide. Los Alamos says strontium is softer than calcium and attacks water more vigorously, and OpenStax adds that barium, the next metal down group 2, reacts hardest at room temperature. The hydrogen shows that these heavier group 2 metals are better reducing agents than hydrogen itself, meaning they give up electrons more easily, which OpenStax reads from the same reaction.

What is strontium titanate?

Strontium titanate is a compound of strontium, titanium and oxygen that Los Alamos says does not occur naturally. Its refractive index, the measure of how strongly it bends light, is extremely high, and it spreads white light into colours more than diamond does. It has been used as a gemstone, although Los Alamos adds that it is very soft.