|Name: caesium||Group number: 1|
|Symbol: Cs||Group name: Alkali metal|
|Atomic number: 55||Period number: 6|
|Atomic weight: 132.90545 (2)||Block: s-block|
|CAS Registry ID: 7440-46-2||Voice: |
|Standard state: solid at 298 K (but melts only slightly||Colour: silvery gold|
|Classification: Metallic||Availability: |
Image adapted with permission from Prof James Marshall"s (U. North Texas, USA) Walking Tour of the elements CD.
Caesium is known as cesium in the USA.
The metal is characterised by a spectrum containing two bright lines in the blue (accounting for its name). It is silvery gold, soft, and ductile. It is the most electropositive and most alkaline element. Caesium, gallium, and mercury are the only three metals that are liquid at or around room temperature. Caesium reacts explosively with cold water, and reacts with ice at temperatures above -116°C. Caesium hydroxide is a strong base and attacks glass.
Nearing Zero cartoon included by kind permission of Nick Kim.
IsolationHere is a brief summary of the isolation of caesium.
Caesium (cesium in USA) would not normally be made in the laboratory as it is available commercially. All syntheses require an electrolytic step as it is so difficult to add an electron to the poorly electronegative caesium ion Cs+.
Caesium is not made by the same method as sodium as might have been expected. This is because the caesium metal, once formed by electrolysis of liquid caesium chloride (CsCl), is too soluble in the molten salt.
cathode: Cs+(l) + e- Cs (l)anode: Cl-(l) 1/2Cl2 (g) + e-
Instead, it is made by the reaction of metallic sodium with hot molten caesium chloride.
Na + CsCl Cs + NaCl
This is an equilbrium reaction and under these conditions the caesium is highly volatile and removed from the system in a form relatively free from sodium impurities, allowing the reaction to proceed. It can be purified by distillation.