Cesium, represented by the symbol Cs and atomic number 55, is not a gas; it is a soft, silvery-golden metal. It is the heaviest stable member of the alkali metal group on the periodic table, making it the most electropositive of all stable elements. This classification means Cesium is eager to lose its single outer electron, which dictates its high reactivity and unique uses.
The Unusual Physical State of Cesium
Cesium is a metal that exists on a narrow threshold between solid and liquid states. While it is technically a solid at standard room temperature, its melting point is remarkably low, approximately \(28.4^{\circ} \text{C}\) (\(83.1^{\circ} \text{F}\)).
Its physical state makes Cesium one of only a handful of elemental metals, alongside Gallium and Mercury, that can be liquid at or near room temperature. Holding a container of solid Cesium in the palm of a hand will often provide enough warmth to cause the metal to melt immediately into a lustrous, golden liquid.
Despite its low melting point, Cesium has a relatively high boiling point of \(671^{\circ} \text{C}\) (\(1240^{\circ} \text{F}\)). The significant difference between its melting and boiling points confirms its identity as a metal. This unusual behavior is a result of the weak attractive forces between its large atoms, which require very little energy to break the solid crystalline lattice.
Extreme Chemical Reactivity
Cesium’s position in Group 1 of the periodic table, the alkali metals, indicates its extreme chemical volatility. As the largest atom in its group, Cesium holds its single valence electron loosely, making it exceptionally reactive. The metal must be handled with caution and stored under specific conditions to prevent dangerous reactions.
The metal is highly pyrophoric, meaning it spontaneously ignites when exposed to air due to its rapid reaction with oxygen. To prevent immediate oxidation, pure Cesium is typically stored submerged in dry, saturated hydrocarbons like mineral oil or sealed in glass ampoules under an inert atmosphere, such as argon gas.
Cesium’s reaction with water is explosively violent. When it comes into contact with even cold water, the reaction generates Cesium hydroxide and hydrogen gas. The considerable heat released instantly ignites the liberated hydrogen, leading to an explosion. This intense reactivity even extends to ice, with which Cesium can react at temperatures as low as \(-116^{\circ} \text{C}\) (\(-177^{\circ} \text{F}\)).
Essential Uses of Cesium
The properties of Cesium are harnessed for highly specialized applications. Its most recognized use is in the development of the Cesium atomic clock, which serves as the international standard for time measurement. These devices use the specific resonance frequency of the Cesium-133 atom—9,192,631,770 cycles per second—to define the second with incredible accuracy.
Cesium compounds also play a significant role in the energy sector, particularly Cesium formate. This compound is used to create high-density drilling fluids for the oil and gas industry, allowing for safer and more efficient high-pressure drilling operations.
The metal’s tendency to easily lose electrons also makes it valuable in photoelectric cells and photomultiplier tubes, where it converts light into an electrical current. In the electronics industry, Cesium is used as a “getter” material in vacuum tubes and light bulbs to absorb and remove trace gases, such as oxygen, that could compromise the vacuum. Furthermore, Cesium has been explored for use in ion propulsion systems, where its high mass and easy ionization make it a highly efficient propellant for spacecraft.