strontium (Sr)


Strontium sample.

strontium and barium flames

Flame test. Strontium flames are crimson; barium are green.

Strontium (Sr) is a soft, reactive, silvery white metallic element named after the town of Strontian in Scotland, where strontium ore was first found. Its principal ores are strontianite (strontium carbonate) and celestite (strontium sulfate). Strontium was first isolated by Humphry Davy by electrolysis in 1808. It is still isolated by electrolysis of the chloride or reduction of the oxide.


Strontium is an alkaline earth metal, found in group II, period 5 of the periodic table. Also in this group are calcium and barium, both of which are similar to strontium chemically. Strontium burns in air and reacts with water. It is used to make special glass for televisions, but is best known for the brilliant crimson color its salts impart to fireworks and flares.


Because strontium is chemically very like calcium, it is easily assimilated by the body, ending up in our bones. Radioactive strontium-90 (half-life 28 years), which is produced in nuclear explosions and released during nuclear plant accidents, is particularly dangerous because it can be absorbed into the bones of growing children.


atomic number 38
relative atomic mass 87.62
electron configuration 1s22s22p63p23p63d104s24p65s2
atomic radius 215.1 pm
density 2.64 g cm-3
melting point 777°C (1,431°F)
boiling point 1,382°C (2,520°F)


Strontium compounds

The two strontium compounds that occur naturally are the sulfate (celestite) and the carbonate (strontianite). Celestite crystals are very beautiful and usually colorless but sometimes have a soft tinge of blue. They can be confused with barite, the mineral form of barium sulfate. Crystals of strontianite, strontium carbonate, are yellowish white in color. They, too, are sometimes confused with the mineral form of barium carbonate, witherite. Flame testing with a small piece of crystal soon settles the confusion (strontium gives a red flame and barium a green flame). Other strontium compounds are manufactured from the carbonate and sulfate.


Strontium hydroxide is made by treating the carbonate with superheated steam. The hydroxide can be used in the refining of sugar. Sugar dissolves in water and therefore water cannot be used to wash it free of impurities. With strontium hydroxide, the sugar forms a white insoluble compound called strontium saccharate. The strontium saccharate is washed free of impurities. The sugar is reformed by blowing carbon dioxide through the saccharate. The sugar redissolves and solid particles of strontium carbonate form. They are then removed from the purified sugar. This method is not universal and in some cases calcium hydroxide is used instead.


Because of its high oxygen content (which helps in burning) and the red color of the flame, strontium peroxide is sometimes used in the burning mixture for tracer bullets. Strontium nitrate, though, is more popular for ordinary fireworks.