We continue to review some of the most important materials in heat treatment and metallurgy.  

Rubidium (chemical symbol: Rb)

Rubidium is a silvery-white, soft metal (Fig. 1), and it is one of the most reactive of all the known elements. It has a density of about 1.5 times that of water, and while it exists as a solid at room temperature, it liquefies (Fig. 2) when warmed slightly (~30ºF).

Rubidium is an alkali metal (the leftmost group of the periodic table) along with lithium, sodium, potassium, cesium and francium. Alkali metals are all shiny, soft and highly reactive and will readily lose their outermost electron to form cations with a +1 charge. In fact, alkali metals are so soft they can be easily cut with a knife, but the exposed surface will tarnish and oxidize quickly in air. While all alkali metals react when exposed to water, rubidium reacts extremely violently (releasing hydrogen) or when exposed to high humidity in the air. As such, it is stored under vacuum.

Rubidium was discovered in 1861 by German chemists Robert Bunsen and Gustav Kirchhoff, who used a new invention, a spectroscope, to examine lepidolite, a lithium-bearing mineral (Fig. 3). Spectroscopic analysis revealed deep-red spectral lines. When further investigated, the chemists were able to isolate samples of a new metal via use of electrolysis.

About 330 pounds of lepidolite was needed for Bunsen and Kirchhoff to extract enough of the new metal to determine its chemical properties. Most rubidium today is obtained as a byproduct of refining lithium. Rubidium played a very minor role in industry before the 1920s. Since then, it has had its most important role in research and development.

Rubidium is used in atomic clocks that measure the most accurate time and frequency standards. Other uses for rubidium are in the manufacturing of photocells, which convert light energy to electrical energy, and as a “getter” in vacuum tubes since its readiness to bond with several gases can remove any trace gases leftover after the tubes are manufactured. Furthermore, some rubidium can be used in fireworks to give the explosions of purplish-red color.

Here are a few important facts about rubidium.[2,5]

  • Atomic number: 37
  • Atomic weight: 85.4678
  • Melting point: 312.46 K (39.31°C or 102.76°F)
  • Boiling point: 961 K (688°C or 1270°F)
  • Density: 1.53 grams per cubic centimeter
  • Phase at room temperature: Solid
  • Element classification: Metal
  • Period number: 5   
  • Group number: 1   
  • Group name: Alkali Metal
  • Electron configuration: [Kr] 5s1



  1. KnowledgeDoor (www.knowledgedoor.com)
  2. Jefferson Lab (https://www.jlab.org)
  3. Chemicool (www.chemicool.com/)
  4. Wikipedia (www.wikipedia.org)
  5. Royal Society of Chemistry (rsc.org)