Cesium
Definition and meaning of Cesium in chemistry.
![Cesium element card: symbol Cs, atomic number 55, atomic mass 132.91, electron configuration [Xe] 6s¹, oxidation states +1, Alkali metal, group 1, period 6, solid at room temperature.](/assets/images/terms/cesium-chemical-element.webp)
Cesium is a highly reactive, silvery-gold chemical element in the alkali metal group. It has the chemical symbol Cs and the atomic number 55. This soft metal is extremely volatile and melts just above normal room temperature.
In more detail
Cesium sits in group 1 of the periodic table as an alkali metal. It is the most electropositive and alkaline of all stable chemical elements. Electropositive means the atom very eagerly gives away its outermost electrons.
The metal easily loses its single valence electron to form a Cs+ ion. Cesium is famous for its exceptionally violent chemical reactivity with liquid water. This explosive reaction happens even at critically cold temperatures around -116 °C.
The reaction produces aqueous cesium hydroxide (CsOH) and highly flammable hydrogen gas (H2). Cesium has an exceptionally low melting point compared to most other solid metals. The solid metal is incredibly soft and easy to cut with a knife.
The pure element readily ignites in the air upon contact with oxygen gas. Chemists must meticulously store it in a vacuum to prevent sudden spontaneous combustion. They can also store it safely under an unreactive noble gas like argon.
Cesium chemical compounds are actually relatively rare in the natural Earth environment. Miners primarily extract the metal from a specific zeolite mineral called pollucite. Geologists generally find this rare mineral buried deep within specific pegmatite rock formations.
Cesium has a uniquely loosely bound outer electron that makes it highly useful. This property is perfect for special applications that require the photoelectric effect. The photoelectric effect occurs when incoming light energy converts into an electrical current.
Key facts
| Field | General Chemistry |
|---|---|
| Symbol | Cs |
| Atomic number | 55 |
| Atomic mass | 132.91 u |
| Category | Alkali-Metal |
| State at room temperature | Solid |
| Melting point | 28.5 °C |
| Boiling point | 671 °C |
Scientists use pure cesium as the crucial foundational element in highly precise atomic clocks. These advanced clocks measure the exact resonant frequency of the cesium-133 atom. This specific frequency directly defines the modern international standard measure of the second. These reliable clocks will not lose a single second in over 100 million years.
Frequently asked questions
Why does cesium possess a distinct gold color unlike most other metals?
Cesium is one of the very few non-gray metals on the periodic table. Its striking golden hue is caused by complex relativistic quantum effects. These effects allow the metal to effectively absorb blue light from the visible spectrum.
How is pure elemental cesium stored safely in a laboratory?
Cesium reacts explosively with trace moisture and spontaneously ignites in the air. Chemists must store it in permanently sealed glass ampoules under a strict vacuum. They can also use an inert cover gas like dry argon.
Related terms
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