We continue to review some of the most important materials in heat treatment and metallurgy.

Cesium (chemical symbol: Cs)

Cesium (Fig. 1) is a rare, silvery-gold, soft, ductile alkali metal that is the softest of all metals in the period table, with a wax-like consistency at room temperature. Cesium is also the most reactive of all metals and is pyrophoric, which means it will ignite spontaneously when exposed to water, even in the amounts of moisture present in air.

Due to this reactivity, cesium must be kept under inert gas/liquid or in a vacuum tube to protect it from being exposed to air and water. During this violent reaction, it will form cesium hydroxide (CsOH), which is the strongest base currently known and will rapidly corrode glass. The most commonly created compounds containing cesium are cesium chloride (CsCl) and cesium nitrate (CsNO3), which are both used in the production of other chemicals.

Cesium was first discovered by German scientists Robert Bunsen and Gustav Kirchhoff in 1860 during the spectroscopic analysis of mineral water from a nearby town. Cesium was the first element to be discovered using the spectroscope (Fig. 2), which was invented by Bunsen and Kirchhoff a year prior.

Under the spectroscope the mineral water showed bright blue lines that did not correspond to any other element known at the time. They named the unknown element after the Latin word “caesius,” meaning sky blue. Upon further use of the spectroscope, Bunsen and Kirchhoff also went on to discover the element rubidium. Today, pure cesium is primarily obtained from extracting it from the mineral pollucite (Fig. 3).

Due to cesium having the second-lowest melting point of all metallic elements, its uses are quite limited. Its largest use in modern times is in cesium formate drilling fluids utilized in oil well drilling. Since cesium combines readily with oxygen, it has uses as a getter material, which combines with and removes trace gases from vacuum tubes.

Cesium also has some uses in extremely accurate atomic clocks. A cesium-based atomic clock will lose one-second per 100 million years. Since cesium has photoemissive properties, converting light to electron flow, it has use in photoelectric cells to convert sunlight into electricity.

Here are a few important facts about Cesium.[2]

  • Atomic number: 55
  • Atomic weight: 132.90545196
  • Melting point: 301.59 K (28.44°C or 83.19°F)
  • Boiling point: 944 K (671°C or 1240°F)
  • Density: 1.93 grams per cubic centimeter
  • Phase at room temperature: Solid
  • Element classification: Metal
  • Period number: 6   
  • Group number: 1   
  • Group name: Alkali Metal
  • Electron configuration: [Xe] 6s1



  1. KnowledgeDoor (www.knowledgedoor.com)
  2. Jefferson Lab (https://www.jlab.org)
  3. Chemicool (www.chemicool.com/)
  4. Wikipedia (www.wikipedia.org)
  5. Chemistry Learner (www.chemistrylearner.com/)
  6. Study.com (www.study.com)