- Caesium (Cs) is a soft, silvery-golden alkali metal with atomic number 55, belonging to Group 1 of the periodic table. It is one of the most electropositive and reactive elements, surpassed only by francium.
- Its electron configuration is [Xe] 6s¹, reflecting the single valence electron characteristic of alkali metals. Caesium atoms contain fifty-five protons, fifty-five electrons, and about seventy-eight neutrons in its most stable isotope, caesium-133 (¹³³Cs). This isotope is stable and naturally occurring, while all other isotopes are radioactive.
- Caesium-137 (¹³⁷Cs), with a half-life of about 30.17 years, is a notable fission product of uranium and plutonium in nuclear reactors and weapons.
- Caesium was discovered in 1860 by German chemists Robert Bunsen and Gustav Kirchhoff through flame spectroscopy. They identified its presence by the bright blue lines in its emission spectrum—the name caesium comes from the Latin word caesius, meaning “sky blue.” This marked the first element discovered using spectroscopy rather than traditional chemical methods.
- Physically, caesium is remarkable for being one of the few metals that is liquid near room temperature, with a melting point of only 28.5 °C (83.3 °F). It has a low density compared to other metals and a high thermal and electrical conductivity. Chemically, it is one of the most reactive metals, igniting spontaneously in air and reacting explosively with water to produce caesium hydroxide (CsOH) and hydrogen gas. Caesium hydroxide is the strongest known base and highly corrosive. Caesium also forms compounds such as caesium chloride (CsCl), caesium nitrate (CsNO₃), and caesium fluoride (CsF), many of which are highly soluble and useful in specialized applications.
- One of the most important applications of caesium is in atomic clocks. Caesium-133 is used to define the international standard of time: one second is defined as 9,192,631,770 oscillations of the microwave radiation corresponding to the transition between two hyperfine energy levels of the caesium-133 atom. These caesium atomic clocks are the basis of global timekeeping, GPS systems, and telecommunications networks. Additionally, caesium compounds are used in photoelectric cells, vacuum tubes, radiation monitoring, drilling fluids for oil extraction, and ion propulsion systems in space technology.
- Biologically, caesium has no essential role in living organisms. Stable caesium is relatively non-toxic in small amounts, but the radioactive isotope caesium-137 is highly dangerous due to its strong gamma radiation and ability to disperse widely in the environment. Exposure to ¹³⁷Cs is associated with increased cancer risk, and it was a major contaminant released during the Chernobyl and Fukushima nuclear disasters.
- Environmentally, caesium occurs in trace amounts in minerals such as pollucite (CsAlSi₂O₆·H₂O), primarily mined in Canada and Zimbabwe. It is rare in Earth’s crust, but its compounds are highly soluble, allowing them to disperse easily in groundwater. Radioactive isotopes, particularly ¹³⁷Cs, pose long-term ecological hazards where nuclear accidents or weapons testing have occurred.
