Caesium metal is highly reactive and is very pyrophoric. In addition to igniting spontaneously in air, it reacts explosively with water even at low temperatures, more so than other members of the first group of the periodic table. The reaction with solid water occurs at temperatures as low as −116 °C (−177 °F). Because of its high reactivity, the metal is classified as a hazardous material. It is stored and shipped in dry saturated hydrocarbons such as mineral oil. Similarly, it must be handled under inert atmosphere such as argon or nitrogen. It can be stored in vacuum-sealed borosilicate glass ampoules. In quantities of more than about 100 grams (3.5 oz), caesium is shipped in hermetically sealed stainless steel containers.
The chemistry of caesium is not very similar to that of other alkali metals, but is more closely similar to that of rubidium, the element above caesium in the periodic table. Some small differences arise from the fact that it has a higher atomic mass and is more electropositive than other (non-radioactive) alkali metals. Caesium is the most electropositive stable chemical element.[note 4]
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