Cesium (Cs)


  • Symbol: Cs
  • Atomic Number: 55
  • Atomic Weight: 132.90545196
  • Element Classification: Alkali Metal
  • Discovered By: Robert Bunsen and Gustav Kirchhoff
  • Discovery Date: 1860
  • Name Origin: Latin: ‘caesius’ (sky blue), for the blue lines in its spectrum
  • Density(g/cc): 1.93
  • Melting Point: 28.44°C
  • Boiling Point: 671°C
  • Appearance: Soft, silvery-gold, highly reactive metal
  • Atomic Radius(pm): 265


Cesium was discovered in 1860 by German scientists Robert Bunsen and Gustav Kirchhoff, the inventors of spectroscopy, when analyzing mineral water. Their analysis revealed bright blue lines in the spectrum, indicating the presence of an unknown element. They named this element cesium, after the Latin word ‘caesius’, meaning sky blue, reflecting the color of its spectral lines. This discovery was among the first to demonstrate the power of spectroscopy in identifying elements.

Relation to Other Elements

Cesium is a member of the alkali metals, located in group 1 of the periodic table, along with lithium, sodium, potassium, rubidium, and francium. Like its counterparts, cesium is characterized by having a single electron in its outer shell, which it readily loses to form a +1 ion, making it highly reactive, especially with water. Cesium has the lowest electronegativity and the largest atomic radius of all stable elements, contributing to its extreme reactivity and low melting point.

Natural Occurrence

Cesium is relatively rare in the Earth’s crust and is most frequently found in the mineral pollucite (cesium aluminum silicate). It does not occur freely due to its high reactivity. The largest deposits of pollucite, which is the primary commercial source of cesium, are found in Manitoba, Canada, and Zimbabwe.


Cesium has several specialized applications:

  • Atomic Clocks: The most accurate atomic clocks use cesium atoms because the frequency of cesium’s electron transition is extremely stable. The “cesium standard” is used to define the second in terms of the International System of Units (SI).
  • Drilling Fluids: Cesium formate brine is used in oil drilling operations as a high-density fluid that can lubricate and cool the drill bit without reacting with the surrounding geological materials.
  • Medical: Cesium compounds, particularly cesium chloride, have been studied for cancer treatment, although their efficacy and safety are controversial and not widely accepted in medical practice.
  • Electronics: Cesium is used in photoelectric cells due to its ability to emit electrons when exposed to light.
  • Space Propulsion: Cesium ions are used as propellant in ion thrusters for spacecraft due to their large mass and ease of ionization.

The discovery of cesium expanded the understanding of alkali metals and showcased the utility of spectroscopic methods in chemistry. Despite its rarity and reactivity, cesium’s unique properties have led to its use in high-precision timing devices, among other applications, highlighting the importance of even the most reactive elements in advanced technology.

Xenon (Xe)

Barium (Ba)