Cesium
Definition and meaning of Cesium in chemistry.
Cesium is a highly reactive, soft, silvery-gold alkali metal with the symbol Cs and atomic number 55. It is known for its extreme chemical volatility and is one of only five elemental metals that are liquid at or near normal room temperature.
In more detail
Located in group 1 of the periodic table, cesium is the most electropositive and alkaline of all stable elements, readily losing its single valence electron to form a Cs+ ion. It is characterized by its exceptionally violent reactivity with water, which occurs explosively even at temperatures as critically low as -116 °C, producing aqueous cesium hydroxide (CsOH) and highly flammable hydrogen gas (H2). Cesium has an exceptionally low melting point and is soft enough to be easily cut with a butter knife. The pure element readily ignites in air, requiring it to be meticulously stored in a vacuum or under an inert cover gas like argon to prevent spontaneous combustion. Cesium compounds are relatively rare, with the metal primarily extracted from the zeolite mineral pollucite found in specific geological pegmatite formations. Its uniquely loosely bound outer electron makes cesium highly useful in specialized applications requiring the photoelectric effect, where incident light energy is efficiently converted into a measurable 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 |
| Year discovered | 1860 |
Cesium is the crucial foundational element used in high-precision atomic clocks, where the exact resonant frequency of the cesium-133 atom directly defines the international standard measure of the second. These atomic clocks are so incredibly accurate that they will not gain or 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 few non-gray metals; its striking golden hue is caused by complex relativistic effects that lower the energy gap between its 6s and 5d orbitals, allowing the metal to effectively absorb blue light from the visible spectrum.
How is pure elemental cesium stored safely in a laboratory?
Because it reacts explosively with even trace amounts of moisture and spontaneously ignites upon contact with air, cesium must be stored in permanently sealed borosilicate glass ampoules under a strict vacuum or an inert gas like dry argon.