- Cesium (Cs) is a soft, silvery-gold alkali metal with atomic number 55, located in Group 1 of the periodic table alongside lithium, sodium, potassium, rubidium, and francium.
- It is one of the most reactive metals and the most electropositive stable element.
- Cesium has one valence electron in the 6s subshell, which it readily loses to form the Cs⁺ ion. Its atomic structure consists of fifty-five protons, typically seventy-eight neutrons, and fifty-five electrons arranged in six shells.
- Naturally occurring cesium consists almost entirely of the stable isotope cesium-133 (¹³³Cs). However, the radioactive isotope cesium-137 (¹³⁷Cs), with a half-life of about 30.17 years, is produced in nuclear fission and is notable for its role in nuclear fallout.
- Cesium is relatively rare in the Earth’s crust, with an average abundance of about 3 parts per million. It is primarily found in the mineral pollucite ((Cs,Na)₂Al₂Si₄O₁₂·2H₂O), which occurs in granite pegmatites. Major deposits are located in Canada, Zimbabwe, and Namibia, with Canada’s Bernic Lake deposit being one of the richest sources.
- The element was discovered in 1860 by German chemists Robert Bunsen and Gustav Kirchhoff through flame spectroscopy, which revealed bright blue spectral lines never seen before. This distinctive blue color inspired the name “cesium,” derived from the Latin caesius, meaning “sky blue.” It was the first element to be discovered using spectroscopy rather than chemical methods.
- In modern applications, cesium is most famous for its role in atomic clocks. The international definition of the second is based on the vibrations of the cesium-133 atom—specifically, 9,192,631,770 transitions between two hyperfine levels of its ground state. Cesium is also used in vacuum tubes, photoelectric cells, and as an ion propellant in spacecraft. Its compounds, such as cesium formate, are used as dense drilling fluids in the oil and gas industry, while cesium salts find application in specialty glass and chemical synthesis.
- Chemically, cesium is highly reactive, even more so than potassium or sodium. It reacts explosively with water, producing cesium hydroxide (CsOH), a strong base, and hydrogen gas. Cesium also reacts vigorously with halogens to form cesium halides (CsF, CsCl, CsBr, CsI) and with oxygen to form cesium oxides. Because of its low ionization energy, cesium readily participates in ionic bonding, and its salts are typically colorless and highly soluble.
- Biologically, cesium has no known essential function in living organisms, though it can be taken up by plants in place of potassium. The radioactive isotope cesium-137 poses serious health hazards due to its high-energy gamma radiation and tendency to disperse widely in the environment following nuclear accidents.
- From an environmental perspective, natural cesium is stable and non-toxic, but radioactive cesium-137 is a major contaminant in nuclear fallout, as seen in the aftermath of the Chernobyl and Fukushima disasters. Due to its chemical similarity to potassium, cesium-137 can be absorbed by living tissues, where it irradiates cells internally. Monitoring and remediation of radioactive cesium are important aspects of environmental safety and nuclear regulation.
