The vast majority of caesium compounds contain the element as the cation Cs+, which binds ionically to a wide variety of anions. One noteworthy exception is provided by the caeside anion (Cs−). Other exceptions include the several suboxides (see section on oxides below).
Returning to more normal compounds, salts of Cs+ are almost invariably colorless unless the anion itself is colored. Many of the simple salts are hygroscopic, but less so than the corresponding salts of the lighter alkali metals. The acetate, carbonate, halides, oxide, nitrate, and sulfate salts are water-soluble. Double salts are often less soluble, and the low solubility of cesium aluminum sulfate is exploited in the purification of Cs+ from its ores. The double salt with antimony (such as CsSbCl4), bismuth, cadmium, copper, iron, and lead are also poorly soluble.
Caesium hydroxide (CsOH) is hygroscopic and a very strong base. It rapidly etches the surface of semiconductors such as silicon. CsOH has been previously regarded by chemists as the "strongest base", reflecting the relatively weak attraction between the large Cs+ ion and OH-. Many compounds are far more basic than CsOH, such as n-butyllithium and sodium amide.
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