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Caesium (₅₅Cs) has 41 known isotopes, ranging in mass number from 112 to 152. Only one isotope, ¹³³Cs, is stable. The longest-lived radioisotopes are ¹³⁵Cs with a half-life of 1.33 million years, with a half-life of 30.04 years and ¹³⁴Cs with a half-life of 2.0650 years. All other isotopes have half-lives less than 2 weeks, most under an hour.

Caesium

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