Strontium (Sr), a soft, silvery metal with atomic number 38, is classified as an alkaline earth metal in Group 2 of the periodic table, alongside calcium and barium. The recognition of strontium as a distinct element involved a series of experimental steps spanning nearly two decades. This process began with the discovery of a peculiar mineral found in the Scottish Highlands.
The Mineral Source and Initial Confusion
The story of strontium begins in the lead mines near the village of Strontian, located in Argyll, Scotland. In the early 1790s, miners discovered an unusual, pale green or brownish mineral distinct from the typical lead ore, galena. This new mineral, later known as strontianite (strontium carbonate), was initially a source of confusion among mineralogists.
The mineral’s physical appearance led many to misidentify it as witherite, barium carbonate, a chemically similar compound. Witherite belonged to the same mineral group and shared similar physical characteristics, making initial differentiation challenging. The element’s name, Strontium, directly honors the geographical origin of the mineral sample, Strontian.
Proving the Existence of a New Element
The initial chemical investigation of the Strontian mineral was undertaken by chemists Adair Crawford and William Cruickshank around 1790. They noted that the material possessed chemical properties different from those of known barium compounds. Their work involved calcination, a heating process that converted the carbonate into a new substance referred to as an “earth,” or strontia (strontium oxide).
The resulting strontia was tested against the two well-known alkaline earth oxides: lime (calcium oxide) and baryta (barium oxide). Crawford and Cruickshank observed differences in the solubility of the strontia compound compared to the others.
Further confirmation came from Thomas Charles Hope, who in 1793 performed more detailed analyses. He observed a striking difference when the strontia compounds were introduced to a flame. Unlike the colorless flame of lime or the yellowish-green flame of baryta, the strontium compound produced a unique and bright crimson color. This distinctive flame test provided compelling evidence that the mineral contained a previously unrecognized chemical constituent.
Isolating the Pure Metal
The final step in the discovery process, obtaining the pure elemental metal, was achieved by the English chemist Sir Humphry Davy in 1808. Davy employed electrolysis, a powerful technique that uses an electric current to break down chemical compounds. He had already successfully used this method to isolate other highly reactive elements, including sodium and potassium.
Davy applied this technique to a mixture containing strontia, likely combining strontium chloride with mercuric oxide to form an amalgam. By passing a powerful electric current from a large voltaic pile through the molten mixture, the strontium was reduced to its metallic form. The resulting strontium amalgam was then heated to drive off the mercury, yielding a small amount of the pure, silvery metal.
This isolation confirmed that the “new earth” identified by Crawford and Cruickshank was indeed the oxide of a metallic element. Davy announced his successful isolation of strontium, along with other alkaline earth metals like calcium and barium, to the Royal Society in June 1808.