Rubidium
Element 37 · Alkali Metal
Rubidium sits below potassium in group 1 of the periodic table and behaves like a more extreme version of it: a single outer-shell electron makes it one of the most reactive of all stable elements. It is soft enough to cut with a knife and melts at just 39.3 °C, so a warm room can turn a solid sample liquid. Rubidium has few bulk industrial uses, but it shines in precision science — rubidium atoms are the working heart of many atomic clocks, and clouds of ultra-cold rubidium atoms were used in the first laboratory creation of a Bose-Einstein condensate, a strange state of matter predicted by Einstein.
Key facts
| Atomic number | 37 |
|---|---|
| Atomic weight | 85.468 |
| Category | Alkali Metal |
| Group / Period | 1 / 5 |
| Block | s |
| Electron configuration | [Kr] 5s1 |
| Common oxidation states | +1 |
| State at STP | Solid |
| Electronegativity | 0.82 |
| Atomic radius | 248 pm (empirical) |
| Density | 1.532 g/cm³ |
| Melting point | 39.3 °C |
| Boiling point | 688 °C |
Occurrence
Rubidium is the 23rd most abundant element in Earth's crust, but it never occurs as a free metal. It is recovered as a by-product of lithium and cesium mining, mainly from the minerals lepidolite and pollucite, where rubidium ions quietly substitute for potassium in the crystal lattice.
Applications
Atomic clocks and frequency standards
Cold-atom and quantum physics research
Photocells and photomultiplier tubes
Specialty glasses and ceramics
Common compounds
- Rubidium chloride
- Rubidium hydroxide
- Rubidium carbonate
Rubidium is so reactive that a pea-sized piece dropped in water explodes, and it bursts into flame on contact with air — samples must be stored under oil or inert gas.
Data source: IUPAC standard atomic weights 2021; NIST physical property data