ElementsWhat is rubidium?
Rubidium is a very soft, silvery metal that catches fire in air and reacts violently with water. It melts at about 39 degrees Celsius.
What is rubidium used for?
Atomic clocks, research in physics, and some special glasses.
Where the name comes from
From the Latin word rubidus, deep red, for the red lines in its spectrum.
Key facts
- Symbol
- Rb
- Atomic number
- 37
- Atomic mass
- 85.468 u
- Family
- Alkali metal
- Period
- 5
- Group
- 1
- Block
- s
- State at room temperature
- Solid
- Melting point
- 39.3 °C (312.46 K, 103 °F)
- Boiling point
- 687.9 °C (961 K, 1,270 °F)
- Density
- 1.53 g/cm³
- Electrons per shell
- 2, 8, 18, 8, 1
- Electron configuration
- [Kr]5s1
- Common oxidation states
- +1
- Electronegativity (Pauling)
- 0.82
- First ionisation energy
- 4.177 eV
- Electron affinity
- 0.468 eV
- Atomic radius (van der Waals)
- 303 pm
- Discovered
- 1861
Place in the periodic table
Rubidium is element number 37. It sits in period 5 (row 5) of the periodic table, in group 1 (column 1), and belongs to the s-block. Just before it comes krypton (36), and just after it comes strontium (38). In the same column, potassium is above it and caesium is below it. Elements in one column tend to behave alike.
Family: Alkali metals.
Physical properties
At room temperature rubidium is a solid. It melts at 39.3 °C and boils at 687.9 °C, so it stays liquid over a range of 649 degrees. Its density is 1.53 grams per cubic centimetre, so a one-litre block would weigh about 1.53 kilograms. By density it ranks 82nd among the 96 elements with a measured value.
Inside the atom
An atom of rubidium has 37 protons in its nucleus and, when neutral, 37 electrons around it. The electrons are arranged in 5 shells: 2, 8, 18, 8, 1. The outermost shell holds 1 electron. Written in full, the electron configuration is [Kr]5s1.
Its electronegativity on the Pauling scale is 0.82. This number shows how strongly an atom pulls shared electrons towards itself: fluorine, the strongest, is 3.98. Removing one electron from an atom takes 4.177 electronvolts (about 403 kilojoules per mole). One mole of rubidium weighs 85.468 grams, so a single gram contains about 7.05 × 1021 atoms.
The ring picture is a simple model that only counts the electrons in each shell. Real atoms do not look like this.
Next steps
- Work out the weight of a compound that contains rubidium with the molar mass calculator.
- Compare it with every other element in the sortable list.
- Test yourself with the symbol quiz.
Figures on this page are from PubChem (U.S. National Library of Medicine), as downloaded on 11 November 2026. See About and data sources.