58 | The History and Use of Our Earth’s Chemical Elements Characteristics
58 | The History and Use of Our Earth’s Chemical Elements Characteristics
Rubidium is located between potassium and cesium in the first group in the periodic table. It is the second most electropositive alkali element and reacts vigorously and explosively in air or water. If placed on concrete on a sunny day, it would melt and then react violently with moist air to release hydrogen with enough heat to burn the hydrogen. If a chunk of rubidium metal is left on a table exposed to the air, it combusts spontaneously. Rubidium must be stored in oil, such as kerosene.
AbundanceandSource Rubidium does not exist in its elemental metallic form in nature. However, in compound
forms it is the 22nd most abundant element on Earth and, widespread over most land areas in mineral forms, is found in 310 ppm. Seawater contains only about 0.2 ppm of rubidium, which is a similar concentration to lithium. Rubidium is found in complex minerals and until recently was thought to be a rare metal. Rubidium is usually found combined with other Earth metals in several ores. The lepidolite (an ore of potassium-lithium-aluminum, with traces of rubidium) is treated with hydrochloric acid (HCl) at a high temperature, resulting in lithium chloride that is removed, leaving a residue containing about 25% rubidium. Another process uses thermochemical reductions of lithium and cesium ores that contain small amounts of rubidium chloride and then separate the metals by fractional distillation.
History Two nineteenth-century chemists, Robert Bunsen (1811–1899) and Gustav Kirchhoff
(1824–1887), collaborated in a search of alkali metals in the mineral lepidolite. Using Bunsen’s new gas burner flame and the new instrument called a spectroscope, they noticed that, when heated or burned, different elements produce distinctive colors of light when transmitted through a prism. When the spectrum of elements in the sun are viewed on Earth via a spectroscope, the element’s light passes through the sun’s light, which (by absorbsion of that element’s light) produces a dark line in that element’s spectrum. In other words, instead of viewing bright lines from the burning element, one sees dark lines because the sun’s light absorbs the element’s bright line. Using the spectroscope, the scientists noticed spectral lines for several colors when they burned lepidolite. The spectral lines for sodium, potassium, lithium, and iron were identified. They also noticed a new distinctive spectral line with a deep red color and in 1861 named its source “rubidium” because it resembled the color of a ruby.
CommonUses Because rubidium is a much larger atom than lithium or sodium, it gives up its outer
valence electron easily, thus becoming a positive ion (oxidation state = Ru + ). Rubidium forms numerous compounds, but only a few are useful. One of the main uses for rubidium is as a getter in vacuum tubes used in early radios, TVs, and cathode-ray tubes. These kinds of tubes work best if all the air is removed, so a getter absorbs the remaining few atoms of air that cannot be removed mechanically by vacuum pumps, thus extending the life of the vacuum tube.
Rubidium chloride is used for the production of rubidium metal, which, in the liquid form, has a high heat transfer coefficient, making it useful as a coolant (along with other alkali
Guide to the Elements | 59 When rubidium gas is placed in sealed glass cells along with an inert gas, it becomes a
rubidium-gas cell clock. Because of the consistent and exact frequency (vibrations) of it atoms, it is a very accurate timekeeper.
Rubidium and selenium are used in the manufacture of photoelectric cells, sometimes called electric eyes. When light strikes these elements, electrons are knocked loose from the outer shells of their atoms. These free electrons have the ability to carry an electric current. Photoelectric cells are used to send a beam across a driveway or garage door. When the beam is broken, an alarm sounds or the door opens.
Rubidium is a very caustic alkali (base) with a high pH value that makes it an excellent reducing agent (highly electropositive) in industry and chemical laboratories.
A unique use is its ability to locate brain tumors. It is a weak radioisotope able to attach itself to diseased tissue rather than healthy tissue, thus making detection possible.
ExamplesofCompounds Because rubidium is a highly reactive metal, it forms many compounds. A few examples follow.
Rubidium carbonate (Rb 2 O 3 ) is used to make special types of glass.
Rubidium chlorides (RbCl) is a source of rubidium metal and is used as a chemical reagent. Rubidium hydroxide (RbOH) is very hygroscopic (absorbs large amounts of water for its weight). It is also an excellent absorber of carbon dioxide. Rubidium hydroxide can be used to etch glass and as an electrolyte in low-temperature electric storage batteries for use in vehicles in the subarctic.
Rubidium also has the ability to form what are called double sulfates. Rubidium cobalt sulfate (Rb 2 SO 4 • CoSO 4 •6H 2 O) is an example of several double sulfates that rubidium has the ability to form. Rubidium cobalt sulfate is a combined rubidium-cobalt compound in the form of ruby-red crystals. Other rubidium sulfate crystal compounds and their colors are rubidium + copper = white; rubidium + iron = dark green; and rubidium + magnesium = colorless.