Rubidium oxide

Rubidium oxide
IUPAC name Rubidium oxide
Other names Rubidium(I) oxide Dirubidium oxide
Identifiers
CAS number	18088-11-4 Y
Properties
Molecular formula	Rb2O
Molar mass	186.94 g/moL
Appearance	Yellow solid
Density	4 g/cm3
Melting point	>500 °C
Solubility in water	Reacts violently to give RbOH
Structure
Crystal structure	Antifluorite (cubic), cF12
Space group	Fm3m, No. 225
Coordination geometry	Tetrahedral (Rb+); cubic (O2–)
Hazards
EU Index	Not listed
Main hazards	Corrosive, reacts violently with water
Flash point	Non-flammable
Related compounds
Other cations	Lithium oxide Sodium oxide Potassium oxide Caesium oxide
Related rubidium oxides	Rubidium suboxide Rubidium peroxide Rubidium superoxide
Related compounds	Rubidium hydroxide
Y oxide (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Rubidium oxide is the chemical compound with the formula Rb₂O. Rubidium oxide is highly reactive towards water, and therefore it would not be expected to occur naturally. The rubidium content in minerals is often calculated and quoted in terms of Rb₂O. In reality, the rubidium is typically present as a component of (actually, an impurity in) silicate or aluminosilicate. A major source of rubidium is lepidolite, KLi₂Al(Al,Si)₃O₁₀(F,OH)₂ wherein Rb sometimes replaces K.

Rb₂O is a yellow colored solid. The related species Na₂O, K₂O, and Cs₂O are colorless, pale-yellow, and orange, respectively.

The alkali metal oxides M₂O (M = Li, Na, K, Rb) crystallise in the antifluorite structure. In the antifluorite motif the positions of the anions and cations are reversed relative to their positions in CaF₂, with rubidium ions 8 coordinate (cubic) and oxide 4 (tetrahedral).^[1]

Properties

Like other alkali metal oxides, Rb₂O is a strong base. Thus, Rb₂O reacts exothermically with water to form rubidium hydroxide.

Rb₂O + H₂O → 2 RbOH

So reactive is Rb₂O toward water that it is considered hygroscopic. Upon heating, Rb₂O reacts with hydrogen to rubidium hydroxide and rubidium hydride:^[2]

Rb₂O + H₂ → RbOH + RbH

Syntheses

For laboratory use, RbOH is usually used in place of the oxide. RbOH can be purchased for ca. US$5/g (2006). The hydroxide is more useful, less reactive toward atmospheric moisture, and less expensive than the oxide.

As for most alkali metal oxides^[3], the best synthesis of Rb₂O does not entail oxidation of the metal but reduction of the anhydrous nitrate:

10 Rb + 2 RbNO₃ → 6 Rb₂O + N₂

Typical for alkali metal hydroxides, RbOH cannot be dehydrated to the oxide. Instead, the hydroxide can be decomposed to the oxide (by reduction of the hydrogen ion) using Rb metal:

2 Rb + 2 RbOH → 2 Rb₂O + H₂

Metallic Rb reacts with O₂, as indicated by its tendency to rapidly tarnish in air. The tarnishing process is relatively colorful as it proceeds via bronze-colored Rb₆O and copper-colored Rb₉O₂.^[4] The suboxides of rubidium that have been characterized by X-ray crystallography include Rb₉O₂ and Rb₆O, as well as the mixed Cs-Rb suboxides Cs₁₁O₃Rb_n (n = 1, 2, 3).^[5]

The final product of oxygenation of Rb is principally RbO₂, rubidium superoxide:

Rb + O₂ → RbO₂

This superoxide can then be reduced to Rb₂O using excess rubidium metal:

3 Rb + RbO₂ → 2 Rb₂O

References

1. ^ Wells, Alexander Frank (1984). Structural Inorganic Chemistry (5th ed.). Oxford: Clarendon Press. ISBN 978-0198553700. 
2. ^ Nechamkin, Howard (1968). The chemistry of the elements. New York: McGraw-Hill. p. 34. 
3. ^ Holleman, A.F.; Wiberg, E., eds (2001). Inorganic Chemistry. San Diego: Academic Press. ISBN 978-0123526519. 
4. ^ Holleman, A.F.; Wiberg, E., eds (2001). Inorganic Chemistry. San Diego: Academic Press. ISBN 978-0123526519. 
5. ^ Simon, A. (1997). "Group 1 and 2 suboxides and subnitrides — Metals with atomic size holes and tunnels". Coordination Chemistry Reviews 163: 253–270. doi:10.1016/S0010-8545(97)00013-1.  edit

Further reading

• "Rubidium Oxide". DiracDelta.co.uk science and engineering encyclopedia. Dirac Delta Consultants. http://www.diracdelta.co.uk/science/source/r/u/rubidium%20oxide/source.html. Retrieved 16 November 2011. 
• "Rubidium compounds: dirubidium oxide". WebElements: the periodic table on the web. WebElements. http://www.webelements.com/compounds/rubidium/dirubidium_oxide.html. Retrieved 16 November 2011. 
• "Rubidium Oxide". fishersci.com. Thermo Fisher Scientific. http://www.fishersci.com/ecomm/servlet/fsproductdetail?LBCID=67207776&storeId=10652&aid=221420. Retrieved 16 November 2011.