Rubidium, a soft, silvery-white, alkali metal with atomic number 37, features a low melting point, ignites spontaneously in air, reacts violently with water, and is not found in its pure form in nature – it exists in minerals such as lepidolite and pollucite.

Rubidium was discovered in 1861, but had limited industrial use until the 1920s. Today, rubidium is considered to be a critical raw material for a combination of reasons, including concentrated foreign supply chains (particularly in China), irreplaceable defense and technology applications, and dwindling global resources – creating a perfect storm of criticality factors.

Reader note: Find the complete history of rubidium here.

20 Interesting Facts About Rubidium

Beyond the above, what else should you know about rubidium? Read on to learn more about this critical commodity!

1. Melting Point: Rubidium melts at just 39.3°C (102.7°F), making it liquid at elevated ambient temperatures and one of only a few metallic elements that can be liquid on a hot day.
2. Radioactive Nature: Naturally occurring rubidium is slightly radioactive due to the isotope Rb-87, which has an extraordinary half-life of 49 billion years – more than three times the age of the universe.
3. Photographic Sensitivity: Commercial rubidium is sufficiently radioactive to expose a photographic plate in just 1 to 2 months, demonstrating its natural radioactivity.
4. Extreme Reactivity: Rubidium ignites spontaneously in air and decomposes water explosively, liberating hydrogen gas which also ignites, making it one of the most reactive metals known.
5. Spectroscopic Discovery: Rubidium was the second element (after cesium) to be discovered using spectroscopy in 1861 by Kirchhoff and Bunsen, identified by distinctive deep red lines in its emission spectrum.
6. Isotopic Composition: Natural rubidium consists of two isotopes – Rb-85 (72.2%) and Rb-87 (27.8%), with 24 additional artificial radioactive isotopes having been created.
7. Electropositivity: Rubidium is the second most electropositive metal after cesium, meaning it has an extremely strong tendency to lose electrons and form positive ions.
8. Atomic Clock Precision: Rubidium fountain clocks are expected to achieve accuracy of 1 part in 10^16, making them among the most precise timekeeping devices ever created.
9. Photoemissive Properties: Rubidium surfaces emit free electrons when struck by electromagnetic radiation across a wide spectrum from mid-ultraviolet through visible to near-infrared.
10. Crystal Structure: Rubidium substitutes for potassium in mineral lattices due to having a nearly identical (10% larger) ionic radius, explaining why it forms no minerals of its own.
11. Abundance Paradox: Despite being more abundant in Earth’s crust than copper, lead, or zinc (78 ppm), rubidium is produced in quantities 1 million times smaller due to its dispersed nature.
12. Dating Tool: The radioactive decay of Rb-87 to Sr-86 is extensively used for radiometric dating of rocks and minerals, providing age estimates for geological samples.
13. Quantum Computing: Ultracold rubidium atoms are used in quantum mechanics research and quantum computing applications, where they can exist in multiple quantum states simultaneously.
14. Resonance Frequency: The resonant frequency of Rb-87 atoms serves as a reference standard for telecommunications, GPS satellites, and frequency oscillators worldwide.
15. Biomedical Tracer: Rb-82, a radioactive isotope, is used in positron emission tomography (PET) for cardiac imaging due to its ideal properties for tracking blood flow.
16. Electron Configuration: Rubidium has the electron configuration [Kr]5s¹, placing it in Group 1 (alkali metals) with a single valence electron.
17. Density Gradient Applications: Rubidium chloride is used in ultracentrifugation to create density gradients for separating viruses, DNA, and RNA due to its high solubility and density.
18. Thermionic Conversion: Rubidium coatings on electrodes enhance thermionic converters by neutralizing space charge, increasing the efficiency of heat-to-electricity conversion.
19. Hygroscopic Compounds: Many rubidium compounds are hygroscopic (water-absorbing), requiring storage in dry conditions or inert atmospheres to prevent degradation.
20. Alloy Formation: Rubidium forms alloys with all other alkali metals, alkaline earth metals, antimony, bismuth, and gold, and readily amalgamates with mercury, demonstrating its chemical versatility.

Final Thoughts

From the ethereal realm of Bose-Einstein condensates, to the practical world of cardiac imaging, from atomic clocks that lose less than a second in a hundred million years, to the frequency standards that synchronize our global communications infrastructure – rubidium demonstrates the truth of materials science: that every element tells a story of the past, a story of the present, and a story waiting to be unlocked – a story that will continue to be told long after us.

Thanks for reading!