Properties and Overview of Rubidium

Overview:

Rubidium (Rb) is a soft, silvery-white metallic element with the atomic number 37 and the symbol Rb. It belongs to the alkali metal group in Group 1 of the periodic table, sharing similar chemical properties with other alkali metals such as lithium, sodium, potassium, and cesium. Discovered in 1861 by German chemists Robert Bunsen and Gustav Kirchhoff using the newly developed technique of flame spectroscopy, rubidium was named after the Latin word "rubidus," meaning "deep red," due to the distinctive red spectral lines observed during its discovery. Rubidium is relatively abundant in the Earth's crust, ranking as the 23rd most abundant element, and is typically found in minerals such as lepidolite, pollucite, and carnallite, often associated with other alkali metals like potassium and cesium. Physically, rubidium is a very soft metal that can be easily cut with a knife. It has a melting point of 39.3°C, just slightly above room temperature, and a boiling point of 688°C. Rubidium has a relatively low density of about 1.532g/cm³, making it less dense than water. The metal exhibits a body-centered cubic crystal structure and is highly malleable and ductile, allowing it to be deformed or drawn into wires without breaking. Rubidium's softness and low melting point are characteristic of alkali metals, which have a single valence electron that is easily ionized, leading to weak metallic bonding and low melting and boiling points compared to most other metals. When freshly cut, rubidium has a bright, silvery appearance but quickly tarnishes in air due to forming an oxide layer.
Chemically, rubidium is one of the most reactive elements, similar to other alkali metals. It reacts vigorously with water, even more so than sodium and potassium, producing rubidium hydroxide (RbOH) and hydrogen gas. The highly exothermic reaction can often ignite the hydrogen gas, leading to a violent explosion. Rubidium also reacts readily with oxygen to form rubidium oxide (Rb₂O) and with halogens to form various rubidium halides, such as rubidium chloride (RbCl) and rubidium bromide (RbBr). It can also react with acids, releasing hydrogen gas and other nonmetals such as sulfur and phosphorus to form sulfides and phosphides, respectively. Rubidium compounds generally exhibit rubidium in the +1 oxidation state, as the element readily loses its single valence electron to achieve a stable noble gas electron configuration.
Safety considerations are essential when handling rubidium due to its high reactivity, particularly with water and air. Rubidium must be stored under an inert atmosphere, such as argon or in vacuum-sealed glass ampoules, to prevent it from reacting with moisture and oxygen in the air. Contact with water or moisture should be avoided to prevent violent reactions, and handling should be done with protective equipment, including gloves and eye protection. In addition, rubidium is mildly toxic, and ingestion or prolonged exposure can lead to health effects such as irritation of the eyes, skin, and respiratory system. While rubidium is not considered highly toxic or carcinogenic, proper safety protocols must be followed when working with the metal and its compounds to avoid chemical burns or other injuries.

Production:

Rubidium is primarily produced as a byproduct of lithium and cesium ores processing, particularly from minerals like lepidolite, which contains small amounts of rubidium. The production process typically involves the separation of rubidium from other alkali metals using fractional crystallization or ion exchange techniques. Rubidium chloride (RbCl) is often produced first and then reduced using metallic sodium or calcium to obtain pure rubidium metal. Due to its reactivity and difficulty separating it from other alkali metals, rubidium is produced in relatively small quantities, with only a few tons being produced globally each year.

Applications:

Rubidium has a variety of specialized applications, although its high cost and reactive nature limit its widespread use. One of the primary applications of rubidium is in research and development, particularly in atomic physics and spectroscopy. Rubidium vapor is used in atomic clocks, among the most precise timekeeping devices available. These atomic clocks, known as rubidium standards, operate based on the hyperfine transition of electrons in rubidium atoms and are used for synchronization in telecommunications, GPS, and other technologies requiring precise timekeeping. Rubidium is also used to produce specialty glass, which enhances the glass's stability and strength. Additionally, rubidium is used in certain types of ion engines for spacecraft, where it serves as a propellant due to its low ionization energy.
In medicine, rubidium isotopes, particularly rubidium-82, are used in positron emission tomography (PET) imaging, a diagnostic tool for assessing cardiac function. Rubidium-82 is a positron emitter with a short half-life of about 75 seconds, making it useful for imaging blood flow in the heart and detecting coronary artery disease. Rubidium's role in PET imaging capitalizes on its ability to mimic potassium, allowing it to be taken up by heart muscle cells similarly. This property, combined with the element's radioactive decay characteristics, makes rubidium-82 an effective tracer for cardiac imaging.

Summary:

Rubidium is a highly reactive alkali metal with distinctive physical and chemical properties. Its softness, low melting point, and high reactivity with water and air necessitate careful handling and storage under controlled conditions. Despite its reactivity and rarity, rubidium is used in various applications, including atomic clocks, specialty glasses, ion engines, and medical imaging. Its production is limited and typically involves separation from other alkali metals in mineral ores. Rubidium continues to be a subject of scientific interest due to its unique properties and potential applications in advanced technologies and research.

See a comprehensive list of atomic, electrical, mechanical, physical and thermal properties for rubidium below: