Rubidium is a chemical element with the symbol Rb and atomic number 37. It is a very soft, silvery-white metal in the alkali metal group. The metal shares similarities to potassium metal and caesium metal in physical appearance, softness and conductivity. It cannot be stored under atmospheric oxygen, as a highly exothermic reaction will ensue, sometimes even resulting in the metal catching fire.
Rubidium was discovered in 1861 spectroscopically by German scientists Robert Bunsen and Gustav Kirchhoff and named after the two prominent red lines of its spectrum. Rb and cesium often occur together in nature. It is found only as an impurity in other minerals, ranging in content up to 5% in such minerals as lepidolite, pollucite, and carnallite. Brine samples have also been analyzed that contain up to 6 parts per million of rubidium.
Characteristics of Rubidium
- It is a very soft, ductile, and silvery white metal. It is the second most electropositive of the stable alkali metals and melts at a temperature of 39.3 °C (102.7 °F).
- It reacts violently with water. As with potassium (which is slightly less reactive) and caesium (which is slightly more reactive), this reaction is usually vigorous enough to ignite the hydrogen gas it produces.
- It has been reported to ignite spontaneously in air. It forms amalgams with mercury and alloys with gold, iron, caesium, sodium, and potassium, but not lithium.
- It is difficult to handle because it ignites spontaneously in air, and it reacts violently with water to yield a solution of rubidium hydroxide (RbOH) and hydrogen, which bursts into flames; It is therefore kept in dry mineral oil or an atmosphere of hydrogen.
- Natural Rb makes up about 0.01% of Earth’s crust; it exists as a mixture of two isotopes: rubidium-85 (72.15 percent) and the radioactive rubidium-87 (27.85 percent), which emits beta rays with a half-life of about 6 × 1011 years. A large number of radioactive isotopes have been artificially prepared, from rubidium-79 to rubidium-95.
- It easily loses its single valence electron but no others, accounting for its oxidation number of +1, although several compounds that contain the anion, Rb–, have been synthesized.