Rubidium (Rb)
Element 37 · Alkali metal · atomic mass 85.468 u · group 1, period 5
Electron shells: 2, 8, 18, 8, 1
Rubidium is a soft alkali metal that ignites on contact with air and reacts explosively with water. Its precise atomic transitions make it valuable for timekeeping.
The first Bose-Einstein condensate, in 1995, was made from rubidium atoms near absolute zero.
- Atomic number
- 37
- Atomic mass
- 85.468 u
- Electron configuration
- [Kr] 5s¹
- Group · period · block
- 1 · 5 · s
- Oxidation states
- +1
- Electronegativity
- 0.82 (Pauling)
- Melting point
- 39.31 °C
- Boiling point
- 688 °C
- Density
- 1.53 g/cm³
- State at 20 °C
- Solid
- Appearance
- Soft, silvery-white metal
- Discovered
- Robert Bunsen and Gustav Kirchhoff, 1861
37 protons, and 37 electrons when neutral
IUPAC standard atomic weight
Alkali metals
What Rubidium is used for
- Atomic clocks and frequency standards
- Specialty glass
- Vacuum tube getters
- Research on ultracold atoms
Atomic structure of Rubidium
A neutral rubidium atom has 37 protons and 37 electrons. Its most common isotope carries about 48 neutrons, which is where the mass number 85 comes from. Change the neutron count and you have a different isotope of rubidium; change the proton count and you have a different element entirely.
Those electrons fill shells from the inside out: 2, 8, 18, 8, 1. In orbital notation that is [Kr] 5s¹, leaving 1 valence electron in the outer shell — the ones that do the chemistry.
3D electron orbitals
See where Rubidium's electrons actually are — a cloud of 26,000 points sampled from the probability density |ψ|², not a cartoon of a lobe.
Where Rubidium sits in the periodic table
Group 1 metals with a single outer electron. Soft, low-melting and so reactive they are stored under oil — reactivity increases as you go down the group.
Discovery and naming
Rubidium was discovered by Robert Bunsen and Gustav Kirchhoff in 1861. Its name comes from Latin rubidus, "deep red", which is also why its symbol (Rb) does not match its English name.
Keep exploring
Atomic weight from IUPAC/CIAAW; physical properties from PubChem (US National Library of Medicine). Values that have never been measured are shown as “not measured” rather than estimated.
Related Calculators
Frequently Asked Questions
What is the atomic mass of Rubidium?
The standard atomic weight of Rubidium is 85.468 u, as published by IUPAC's Commission on Isotopic Abundances and Atomic Weights.
How many protons, neutrons and electrons does Rubidium have?
Rubidium has 37 protons, because the atomic number is the proton count and that is what makes an atom Rubidium at all. A neutral Rubidium atom has the same number of electrons, 37. The neutron count varies by isotope; the most common one has about 48, since 85 − 37 = 48.
What is the electron configuration of Rubidium?
The electron configuration of Rubidium is [Kr] 5s¹. Its electrons are arranged 2, 8, 18, 8, 1 across 5 shells.
Is Rubidium a metal?
Yes. Rubidium is classified as a alkali metal, on the left side of the table.
What group and period is Rubidium in?
Rubidium is in group 1 (the alkali metals), period 5, in the s-block. The group is its column and the period is its row.
What is the charge of Rubidium?
Rubidium most commonly forms a 1+ ion. It has 1 electron in its outer shell and loses it readily, which is what the group number tells you.
What is the melting point of Rubidium?
Rubidium melts at 39.31 °C and boils at 688 °C. That puts it in the solids at everyday temperatures.
What is Rubidium used for?
Atomic clocks and frequency standards, Specialty glass, Vacuum tube getters, Research on ultracold atoms. Rubidium is a soft alkali metal that ignites on contact with air and reacts explosively with water.
Who discovered Rubidium, and where does its name come from?
Rubidium (Rb) was discovered by Robert Bunsen and Gustav Kirchhoff in 1861. The name comes from Latin rubidus, "deep red".